Spunky - I hope you get this info quick!
The three rules of cheating - DENY, DENY, DENY!!!!!!
BTW - You wont need to kick your "buddies" ass. Just deny the shit to your girlfriend, and explain that this jackass just wants to get to her, and is fucking you over to do it.
Next - Tell everyone in the fucking world that will listen, that this guy lied about the other chick to screw you over. After that he wont have any friends left, and nobody will listen to anything he says.
That said, if you already agreed with the story to your girl, your screwed. At that point play honest, say you were drunk, and it will never happen again because she means too much to you and your not interested in other women.