you should feel ashamed, and sounds to me, it was like an accident for you, she might've forced you to have sex with her and you just felt like it (i can understand), its your fault but its more of her fault, so what you could do is, admit it, just describe the situation, how it happened.. she'll be broken for a while but forgive you (or even not) but she'd never forgive the friend cuz shes a total b*tch for doing that, and if you tell the truth, atleast you'll save your girlfriend from this type of friend! ciao~