My friend is being horribly stalked and harassed online by a non-US citizen who currently outside of the US, but is causing her & her family serious concern for safety & wellbeing. He appears to have a mental disorder. What would be the appropriate place to report this to? Thank you!
Normally, immigration officials wouldn't be involved in a case like this one.
Normally, criminal court actions are only effective if the defendant can be arrested by a country where the crime was committed or where the crime was directed, or by a country that will extradite the individual following a request made through diplomatic channels.
One way to get there would be to complain to police where the person is located. If you know precisely who it is, and where the person is, this might produce action since someone like that might be a threat the local police care about on the theory that this person might be dangerous to other people as well. Unfortunately, the cases where you are most like to have this information (e.g. stalking and harassment from an ex-spouse or lover) are the cases that local law enforcement where the offender is located is likely to take least seriously.
Another would be to complain to a law enforcement agency (e.g. the FBI) that is able to get the attention of diplomats to bring charges and request extradition, in cases where extradition is available (which depends upon the laws of the offender's country, normally this is only available if the offense would be a felony in the place where the offender is located).
As a practical matter, the nature of the conduct would have to be pretty intense and amount to some sort of terroristic threat before action was taken criminally through diplomatic extradition channels.
Normally, a civil action is only effective if the person to be bound by the order is served with process issued by a court that can have jurisdiction over the individual. Typically, U.S. law would give U.S. courts jurisdiction over wrongful conduct directed at Americans, but if the individual involved is not in the U.S. enforcing the order of a U.S. court might be difficult.
In a domestic situation, one would seek a restraining order or protection order against the offending individual. This could be useful if that person later appears in the U.S., but getting service of process on that individual could be hard.
It might be possible to get an order from a U.S. court applying to the online entity facilitating the harassment by the foreign person to censor that person, but you end up with a whack-a-mole situations where getting an order is hard and as soon as one is obtained the offender can move to another online means of conducting harassment or can operate under an alias that can't definitively be tied to the original offender.
Also what constitutes "horribly stalked and harassed online" is too vague to know if the conduct would constitute action for which a criminal or civil remedy is available. Likewise, it is unclear who certain you are about the identity or location of the offending individual. No legal action is possible without clarity of both of these things.
Bottom line: From a practical perspective, you are pretty much limited to self-help, defensive measures (locking doors, not being alone, not giving out privacy disclosing information) that cramp your life, and to complaints to the administrators of the online forums where this conduct happens.