Help!!!

I need ways to catch a boy cheating.
I know it might sound weird but my friend has been in an a abusive for the year and a half and each time she tells him that they need to break up he gives her an answer that leaves her without any response. he gets rlly angry when she gets close to any boys and isn't the nicest when it comes to controlling his emotions. Apparently in the first 6 months things were going rlly well, but I obviously knew that there was gonna be a catch(probably cause I have trust issues that came from yaoi mangas) and then he started being all insecure and stuff and that's when i decided to pull the trigger, but that didn't work cause she thought that he was acting cute. Then as time went on things became worse and then she finally realised it was time to break it off, but that didn't work. Back to current day, I decided that we need to find a way to catch him cheating, but the problem is I can't think of any good ways.
Btw he has cheated before but we had no evidence to support it.
So if u have any suggestions plse tell me. Thank u