You aren’t responsible for his decisions – he definitely should feel guilty. However, if you are a decent person you will eventually come to realize that by doing this you are implicit in an act that will hurt this other person, this girlfriend of his. I know you don’t have to care for her, she’s just a stranger but the karma bus will get you. And I am not holier than thou. I’ve done this – slept with a man when he was married to his wife and she had no idea…but their marriage was dead and he didn’t want to be with her. Once we slept together, he told her everything and divorced her. Sleeping with a man when he’s engaged…what kind of marriage will he have? When my husband was engaged to his first wife, he slept with another girl (who is now, ironically, bound to be a Unitarian Universalit priest but people change)...he should have know it was a red flag and that his marriage wouldn’t last…when I was engaged to my first husband, I cheated on him as well (denying it all in my head)...I should have known…it didn’t last.