It's just a kiss. Ok, you'd feel better if she hadn't told you - but she apparently needed to get it off her chest because of all the guilt she was feeling. Guilt = she obviously values your relationship and doesn't want anything to ruin it. So while ignorance is bliss, getting to know about it means that she cares.
She made a mistake - we all do. At some point you might find your self in the same situation, then you'll be the one with the guilt gnawing you.
Had I done that to you (baby, snookums), I'd also have told, knowing that receiving the news sucks bollocks, and being selfish enough to hurt you with that information, but hopeful that my truthfulness would enable us to build on our relationship even further. I did that once to a girl, back in the day, 1999. But because I felt like shit, I ended the relationship and spent the next 2 years or so single. I ended up regretting that decision for a long time.
Meet up with her as soon as you possibly can. Preferrably.. ..now.
1) Express your dislike/anger/frustration for what has happened, but immediately after that, go to step 2
2) Forgive
2) It was probably her "old friend" that made all the advances - you should tell your girlfriend that if she doesn't set him straight, you will.
3) Do your best to not let this come between you. I know it hurts, but it's just a kiss. Probably just one kiss and that's all. If she still wants to be with you, stay by her side and let time heal all wounds. If this was the first time, then it probably will be the only time.
Go see her.
I need around tree fiddy.