here's how i see it....
1.) you gave into temptation. i can't blame you, if she was pushing for it or willing as she obviously must have been then its hard to say no. while its not the right thing to do, you're only human, and everybody makes mistakes. everybody makes a lot of mistakes....that's life man. making mistakes, and learning from them.
2.) she let it happen. i'm sure it isn't the first time a guy has flirted with her or like made a move and tested the waters with her but she probably hadn't given in before. she made a mistake as well, and in my opinion it is up to her to tell her boyfriend about it if she chooses to. If she does tell him, or if he finds out, do not be surprised if you have to deal with a very angry person who probably will want to hurt you.
so what do you do?
- you learn from this experience, hopefully next time you will think with your head and not your dick and you will realize the consequences that could result from you doing that are probably not worth the pleasure you might get. while it will be hard to do that in the moment, at least you'll have been the before and be able to think it thru a little better. So i hope that kid doesn't come after you, or if he does that you have good interpersonal skills and can work it out instead of getting in a fight. Just think it out man, that's all you can do. But don't let it kill ya, it happened. its over with. move on.