Tough predicament I've been put in......

Ok today was a good day. Got to see my freind get slaped by this girl that hates him cause he went around and told every1 that she raped him lol .. But then I came home started talking to this girl who's one of my best freinds....ok I won't use names in this to protect the people but listen to this hole im in. This girl told her boyfreind in december that she hooked up with me when we never did (all though i always wanted to) but I really wouldn't have cared for the fact that I never thought Id meet him.Until fri then sat and I think him and his freinds are kool guys. But see at this time I didn't know bout wat she had said to him about me, until today, and now he like dislikes me lol wtf. not really, I could give 2 shits but still it's just the fact. Then I find out why see used me as an excuse to he boyfreind. She fucking hooked up with her BF's bestfriend haha. Then she tells me that I was the only person she thought she could trust.(which is true im a very trusting guy).Now heres my delimea, Do I tell him the truth to rectify the situation, and lose her as a good friend? Or do I keep the cat in the bag and just say fuck it I don't care what he thinks?