Oh my God why? If she's your bestfriend, why? I meanyou put two relationships in jeapordy, your friendship and their relationship, and I've heard of ppl killing theirselves over less. Ik it was a mistake but was u thinkin?!?!? Girl u screwed up, but idk what you shud do. Evaluate her as a person and yourself your friendship and how much she reallymeans to you and ask yourself, "can she handle this"? But you shud tell her its goin to come out somehow and the longer you hide it the worse her reaction will be, unless its the other way around, but in big situations like this think super hard the answer is within you. Idk your friend or her personality and how strong your relationship is so I can't give you all the info. Sorry about how I started out scolding and stuff but thts really horrible, but I hope all goes well, again, sorry for my extremely strong reaction.