Warning: I am not a lawyer, and the quotes from the Mexican and Guatemalan criminal codes do not belong to official translations, but are slightly edited (by me) results of Google Translate.
First of all, as @Martin Bonner suggests, under many jurisdictions ordinary citizens are not legally required to report the vast majority of crimes. For example, ordinary citizens in Italy are only legally required to report crimes mostly related to terrorism, explosives, and money counterfeiting.
I didn't find any specific information concerning this issue under the US, Mexican, Philippine, or Guatemalan law. However, if reporting such crimes is mandatory in any one of these countries, we can safely assume that the crew did report them - lest they get in trouble with local authorities - , only they didn't report them immediately.
Crimes related to possession - of illegal firearms, drugs, and so on - don't need to be immediately reported. The crew might have reported these crimes to the police after completing the documentary, specifying names, identikits, amount of firearms, location,etc. or (but it's a riskier option) even while filming the documentary. This is not, to my knowledge, a privilege of documentary crews.
Think this way: if you are invited by friends at a party, and find out that they store illegal firearms and drugs, you could call the police and immediately report the crimes, but this is very risky. After all, criminals dealing with firearms typically know how to use them and are often ready to use them at any given time. Your best bet would be to come back home after the party, and call the police.
As for different crimes such as battery, assault, robbery and murder, the situation becomes more complex, as these crimes could be immediately reported in order to prevent their execution.
However, I cannot think of any jurisdiction under which you are required to testify about crimes within a prescribed amount of time, because this would lead to loopholes.
As an example, consider a bank robbery. The criminals threaten with a gun all the customers and employees of the bank and hold them hostage for X hours. If the time interval prescribed by the law elapsed during the robbery, all the customers and employees could be deemed guilty, but that is not the case.
Another example could involve a victim of kidnapping. Assume that A is kidnapped by B and forced to assist to the murder of C. A is kept hostage for days, weeks or even months. If the time interval prescribed by the law elapsed before A getting freed, A could be deemed guilty as an accomplice or accessory to B for the murder of C, which is clearly not the case.
Basically, as long as you don't actively participate in the execution of the crime, don't hide the criminals nor conceal weapons and other objects used for the execution of the crime, and report the fact to the police, you should be fine.
Let us check this fact in the particular case you described.
Under the Philippine Revised Penal Code, which you can find at http://www.un.org/depts/los/LEGISLATIONANDTREATIES/PDFFILES/PHL_revised_penal_code.pdf:
Art. 19. Accessories. — Accessories are those who, having knowledge of the
commission of the crime, and without having participated therein, either as principals
or accomplices, take part subsequent to its commission in any of the following
By profiting themselves or assisting the offender to profit by the effects of the crime.
By concealing or destroying the body of the crime, or the effects or instruments thereof, in order to prevent its discovery.
By harboring, concealing, or assisting in the escape of the principals of the crime, provided the accessory acts with abuse of his public functions or whenever the author of the crime is guilty of treason, parricide, murder, or an attempt to take the life of the Chief Executive, or is known to be habitually guilty of some other crime.
I don't think that the documentary crew concealed or destroyed the body of the crime, or the weapons being used, so case 2 can be ruled out. Case 3 can be ruled out as well, provided that the crew didn't conceal the principals of the crime. This leaves us with case 1.
Did the crew profit by the effects of the crime?
Considering property crimes such as robbery, A profits by the crime committed by B if A receives a part of the stolen good or if A earns money if the crime has been committed by B. Assuming that the crew didn't receive a part of the stolen goods, we need to understand if they earn money if the crime has been committed.
Again, I'm not a lawyer, but I think this is not the case. Filming crews usually get paid per hour/day, not per audience share of the TV show. Therefore, even if the audience share increases as a result of many crimes being shown, the crew doesn't receive additional amounts of money. As such, the crew cannot be considered as accessory for the crimes committed by the criminals, at least under the Philippine law.
Now, let us consider the Mexican law and how it defines the crimes of illegal arms sale and selling stolen goods.
First of all, let us quote Article 13 of the Código Penal Federal:
Article 13.- The authors or participants in the crime are:
I Those who agree or prepare its realization;
II those who perform it on their own;
III Those who perform it along with other people;
IV Those carrying it out on the basis of another;
V Those who intentionally provoke another to commit it;
VI Those who fraudulently aid or abet another to commit it;
VII Those who help the criminals after the execution of the crime, as a result of a promise previously made to them;
VIII those involved in the commission without prior agreement, when it cannot be pinpointed the result that each one produced.
Furthermore, Article 162 of the Código Penal Federal specifically describes the categories of people who can be deemed guilty of crimes related to possession and trade of illegal weapons:
I. Those who import, manufacture or sell the weapons listed in Article 160; or give them or traffic with them;
II Those who sell pistols and revolvers, lacking the permits required;
III Those bearing prohibited weapons under Article 160;
IV Those collecting weapons without lawful purpose or without permission;
V Those carrying a weapon specified in Article 161[pistols and revolvers] without permission.
Article 368 bis deals with selling stolen goods:
Article 368 Bis.- He who ,after the execution of theft and without having participated in it, possesses, disposes of or traffics, acquires or receives instruments, objects or products robbery, knowing this
circumstances and if the intrinsic value of these exceeds five hundred times the salary, shall be punished with three to ten years of prison[...]
If the crew didn't receive nor give away stolen objects, they cannot be deemed guilty under Article 368 bis.
It seems to me that the crew cannot be classified in any of these categories described by the three articles and, as such, that they cannot be considered liable for the crimes perpetrated by the dealers under the Mexican law.
I didn't find much about these crimes as defined in the Guatemalan criminal code (http://www.un.org/Depts/los/LEGISLATIONANDTREATIES/PDFFILES/GTM_codigo_penal.pdf), but I did find the definition of authors and accomplices under this jurisdiction:
Article 36. Authors are:
Those who take a direct part in the execution of the acts of crime.
Those who force or directly induce others to commit.
Those who cooperate in the commission of the crime, either in its preparation or in its execution, by performing an act without which the crime would have not been committed.
Those who, having agreed with another or others for the commission of a crime, are present at the time of its completion.
Article 37. Accomplices are:
those who encourage others in their decision to commit the crime.
those who help or cooperate with the authors after the crime.
those who provide appropriate means for the execution of the crime.
those who served as liaison or acted as intermediaries between the participants to obtain their participation in the crime.
Once again, it seems that the filming crew cannot be considered author nor accomplice of the crimes perpetrated by the dealers under the Guatemalan law.
Finally, let us consider the American criminal law. Under Title 18 of the United States Code (http://uscode.house.gov/download/download.shtml):
(a) Whoever commits an offense against the United States or aids, abets, counsels, commands, induces or procures its commission, is punishable as a principal.
(b) Whoever willfully causes an act to be done which if directly performed by him or another would be an offense against the United States, is punishable as a principal.
§3. Accessory after the fact
Whoever, knowing that an offense against the United States has been committed, receives, relieves, comforts or assists the offender in order to hinder or prevent his apprehension, trial or punishment, is an accessory after the fact.
As we can see, even under the American law the filming crew cannot be considered as principal nor as accessory after the facts.
Hence, if the filming crew has fulfilled any other obligation prescribed by the laws of these countries (such as reporting the crimes to the police if it is legally required), they cannot be deemed guilty of any offence committed by the criminals.
As an aside, you shouldn't try to follow underground groups around the world in order to document illegal activity if you aren't sure that the criminals wouldn't kill or hurt you OR if you don't know whether you are breaking or not the laws in the countries you are filming OR if you end up assisting, even unintenionally, the criminals.... OR, basically, if you're not part of the filming crew of a TV channel and are backed up by lawyers and, if anything goes wrong, by the local authorities and by the Embassy of your country in the States you wish to visit.