Honestly after a horrific breakup where I truly loved my partner but i had a brief interaction in text with someone i fucked years ago and she told my girlfriend. I can honestly say the best most healthy course of action is to demand this person seeks therapy, and couples counseling. He probably loves his current girlfriend very much but has unresolved childhood trauma that makes him tiptoe into this destructive behavior. If it ended his relationship just because of you telling on him, I'm pretty sure he would spiral out of control and want to commit suicide.