When two people are in a relationship, they should be committed to one another. If they are not I would not call it a serious relationship. If he did make advances toward you, tell the girl and see what happens. I'll bet she'll pass on the friendship with you and go for the relationship with him. If you made passes at him, I think he should tell her and then she'd completely alianate you, and look at you as a trespasser from here on out. However, if he hit on you and you tell her, and there is already distrust in the relationship, perhaps that will be the final push she needs to leave, but she will still not really want to be around you because you were the bearer of the bad news. Of course I could be wrong, or I could be speaking from experience of being with someone who wasn't very faithful. Your call, but be prepared for it to end poorly.