Well I hope you didnt kiss him. hey dont let this guy ruin your freindship with her thats what happens on earth. if you did kiss him and cheating on you tell your freind the truth and fess up thats how I roll.
If he was interested in someone else(dating etc...) the blame is on him. The person he was with first, deserves to know that he's playing the field. It's a shame people usually aren't mature enough to lay the blame where it belongs. However you should do some digging and be more informed about the people you are considering having a sexual relationship with. Hopefully a lessoned learned for your future.