I say tell her. If one of my boyfriend's friends knew he was cheating on me I would want them to tell me. I would not give up that person as a friend I would ditch my boyfriend. I say if your friend with his girlfriend then you are a horrible friend if you don't tell her. He could give her an STD you have a moral obligation to tell her. If you have a heart tell her. She will be better off if you do. If you lose your friend over it then he is not a true friend. If he is cheating on her its clear he is not happy. You are doing both of them a favor.