"How can someone do this...a good friend of mine has been going out with his girlfriend for well over a year now. Last Friday was our senior prom. Friday night, after the prom, she cheated on him. I can't believe that...cheating is bad enough, but on freakin prom night? That's so horrible. That pisses me off so much."
Well, technically, she isn't obliged or committed to him.
However, putting aside the technical part, if your friend were a wise guy, he would be rejoicing this happened. He would be rejoicing 'cuz he got to see the true nature of his so-called gf at an early stage.
Now, he would be a true simpleton if he stuck with her despite her true colours. And if he does that but comes whining, then I'm set to stamp him the title "wimp/sissy".
So, let's hope he gets wise here.
Hey, that's my sincere opinion, so don't be trying to start trouble with me. Hehehe!
Edit: Made a major typo! 