I was thinking the same thing - it's overly simplistic to say "you have only been together six months" from the outside, when you have no idea what the quality of those six months has been. Yes, she kissed someone else, and that's serious - but nobody except her and the dude even know the circumstances.
My personal opinion? Keep talking to her and see how sorry she really is, and what the circumstances were. She may just be sorry now because she was caught, or she may be truly and deeply apologetic. Only time and introspection on her part will tell you the answer to that.
I think the main point of bringing up the young age is it can show a lack of experience in relationships. I think you make a great point, its a really good learning experience not only for this problem but the others that come with a commitment.
My biggest concern is that they've only been together for 6 months. How long were they together BEFORE they moved in. I think thats what seperates this from all of the examples people give of "my wife and I met in...". For every example of that there are countless numbers of saps who weren't ready to handle an adult relationship. I don't think age effects the seriousness of a relationship but the move to live together seems really premature, and may have sped up the relationship unintentionally. You see this all the time, from people of all ages. It doesn't mean their not in love or aren't perfect for each other, it just means relationships are really tough and unexpected things come up all the time.
Sorry to say this buddy but I'm guessing the odds are better she'll end up with this friend than you man. Good luck to ya, I hope you things work out for the best.