That the cop claims to be your friend is not more illegal than a salesman claiming that he has "the best offer" for you because he likes you (in fact didn't you see any film about the good cop/bad cop routine?)

The term you are looking for is [Entrapment](https://en.wikipedia.org/wiki/Entrapment). The (very simplified) basic idea is that police officers can promote the comission of a crime to catch criminals, but cannot provoke it, the difference being that their persuassion should not cause otherwise innocent people to become criminals.

As I saw it explained (just for illustration purposes, it is not that you are safe when the cop insists a third time):

* Legal:

 Cop) Oh boy! The place where I work is full of cash and they don't even have alarms or store it in a safe box. If someone helps me, we could go this night and take all of it. Are you interested?

 Target) I don't know.

 Cop) Trust me, it will be easy, nobody is there at night and it will be just a couple of hours.

 Target) Ok, count me in.

* Illegal

 Cop) Oh boy! The place where I work is full of cash and they don't even have alarms or store it in a safe box. If someone helps me, we could go this night and take all of it. Are you interested?

 Target) I am not a thief.

 Cop) Come on, it will be easy, the place is insured and nobody will be hurt.

 Target) Not interested.

 Cop) We can get 5000 US$ each one, just for a night of work. Didn't you told me that you had troubles with your bank? You could solve those overnight!

 Target) Maybe you are right...

In any case, this is generic information only, entrapment is difficult to prove and will depend on the views of the judge/jury so, no matter how enticing that criminal offer is, just don't do it.