The best thing you can do is own up to it. say "yeah, that was f cked up of me messing around with your friend, and I dont blame you if you dont want to talk to me, so I'll leave you alone." BUT...dont say this apologetically. Say it maturely, and matter of factly. You didnt cheat on her, because it wasnt a relationship, but you hooked up with her friend, and that is disrespectful and hurtful, and you have to own up to it like a man, and walk away. If she forgives you, she forgives you, if not, you deserve to lose her anyway, so move on.
