The most regretting thing you can ever do to a friend you like and who likes you back? PLEASE HELP!

Okay I am writing a book on romance about two close friends who are boy and girl and the boy becomes a celebrity and enters a hot boy band and it's kinda hard for the friends to see each other so one day he comes back and everything is fine except for a few Hollywood side affects and the girl thinks their still cool but one day she finds out something REALLY BIG that she never knew he did and is really hurt by it and she's upset about why he did it.
Now see my story is much more interesting than this but I'm just rushing through so yeah, anyway that "Big secret" is what's stomping me. Now I've never done anything so bad that it broke someones heart because I'm WAY too nice for that so I'm just trying to figure out what that is. The boys persona is nice, outgoing, goofy, loud, crazy, and relate like so if that helps. Please help me. I'll take anything. Thank you for your time. :D