I have been in this situation 2x. The first time, I ratted out the woman who introduced me to my wife, because her husband had become a very good friend of mine. I was also offended because she was using times that she knew I was with him to do her deal so I felt a little used. Have been thanked for years on this one.....



The second time, a friend and I hired a PI and busted another friend's wife, because we knew he would never believe us. She was cheating with his best friend and a neighbor. We told him, he moved out, moved back in a month later and we don't speak anymore......

If it were me and I was equally friends with both, I wouldn't tell. But I wouldn't hang out with them anymore, either.