In terms of Islam:
It is Islamically impermissible to accuse someone of any sort of negativity without evidence which cannot be proven in a court of law. Following are all impermissible acts:
You are always supposed to think best of a Muslim, doesn't mean that you trust them with the best of your secrets or your money (understand the subtlety).
There are two exceptions in Islam in which case I have to report my "suspicions" and they are:
In fact you are supposed to DISTRUST a Muslim when it comes to these two issues as per the Madhab of Imam Abu Haneefa (RA).
If a woman is trying to marry "In Need of Teaching" then her and her wellwishers are supposed to think that "In Need of Teaching" is a JACKASS until evidence is proven to the contrary! Everything he says, has to be proven...
P.S: This is for people of course and I am not discussing state agencies like the Police, criminal investigations, espionage, intelligence etc
P.P.S: Many Muslims fallout with their best friends over marriage. The Sister or her family contacts you about (your friend) and if you follow Sunnah and your friendship is superficial it is possible that your relationship will break.
This cannot be undone and I am sure it will be greatly appreciated.