Trust No One.
Well, not really, that's a little extreme.
Dump your friend, no excuse in the world is good enough ( like, "she came on to me, " we were drunk" , etc. )
Dump the girl, same reason. They could have stopped at any time, but thought they could get away with it. They will try it again.... human nature.
You will always be second guessing their actions, always be worried about who she's with. Don't put yourself through that. It keeps you awake at night, makes you crazy, and will turn you into a psycho-jealous-overprotective-never trusting boyfriend for some future girlfriend.
The part of you that cares for her wants you to give her a chance to redeem herself. You will try to talk yourself into it, and it will win over the "get rid of her" part of you.
Once that happens, history will repeat itself.
Don't have any phone contact or email contact with them. Don't do or say anything you will later regret, because they will twist your words and actions to make you look like the bad guy. Just cut them off....end of story, start fresh after you take a break from relationships, and DON'T jump into another one right away.
Best of luck, brother pizzaman.