My close friend's cheating boyfriend (who's also my friend)

So I was at a party hosted my guy friend who is dating one of my closest friends. My girl friend got really drunk at the party and ended up having an argument with him...(again). Now she hasn't been happy for a while especially since they haven't had sex in 6 months and she's frustrated, feels rejected and ugly. The relationship itself has become an abusive one on both sides, it's on the rocks but still our friend refuses to give up on it. She says she doesn't know what's going on with him, and that generally he has a lower libido than she does but she sometimes complains that he might be cheating on her. A few months ago, she found a used condom in his bin and when she confronted him about it he acted weird about it and claimed he used it to masturbate or his housemate dumped it in there he can't remember, it was really sketchy. Me and his other house mate, whose her best friend got very suspicious about it and feel our friend is just too delusional about their relationship. The night after the party, her best friend and I had a long chat and she revealed to me that she knew our friends bf had cheated on her more than a couple of times and that she thought he was gay because on more than one occasion she caught him in the act with other men as well as girls. Our friend is very fragile and she's given up so much to be with guy, but she needs to find out so she can finally break up with him. Her best friend has known of this for over a year now, she may lose her as a friend when she tells her. We don't know how to go about this without bypassing the betrayal our friend will feel. We're not foolish about the situation but we want to make things easy as can be allowed for her when she's inevitably goes through a really rough time with this. Any suggestions?