i understand how you're feeling, and i do believe it is possible for this to be a 'one-time thing', but i would ask more questions. first of all, it seems like you're really clinging to the idea that she would never do this, or initiate this. Is that denial, or is that a real appraisal of the situation? Seems to me that if she was joking with a friend about it, she's not exactly completely ashamed of herself.
it's best for relationships to develop on solid ground and honesty. this includes accepting the possibility that she might have found herself attracted to this other guy. maybe she was very willing to kiss him, and maybe it was more than a kiss that actually happened. unless she tells you, you'll never really know. unless she feels like she can be honest with you without you wigging out, she will decide it's better just to stroke your ego and minimize the incident.
whatever happened between her and him, it was a short-lived incident. if she wants to be in a monogamous relationship with you, and more importantly, if you still want her in your life, then you should talk out the circumstances of what happened and then forgive her. Don't forget that you might also have to examine your own conduct in the relationship...maybe there's something you need to apologize for too.
in my experience there's always two sides to a story, and the best way to sort it out is to be strong enough to face both sides without condemning her or deciding to be blind to the facts.