i think it depends on where you are with stuff and the situation. me personally, probably not. but.... 1. if its totally casual - no involvement 2. you feel that she's gonna do it with someone else anyway 3. you're totally uninvolved with the situation (don't know the guy, etc.) i could see it. i mean the responsibility is really with her. reverse situation, the cheating husband is the responsible party, not the girl he picks up for the one night stand. its even possible that if she has sex outside her marriage this one time, it might help her situation - get her to talk / confront the guy, realize that she made a mistake, etc. those are her problems though, not yours....