From the sound of it some guy stole your girl from you. If that is the case, the girl was no good for you anyway. Sometimes we need certain circumstances to present themselves to see who is loyal to us and who isn't. Forgive the guy and just learn to tolerate him and forget about that chick. Move on and find a better girl.
Im a firm beleiver in puttin someone in their place. You say hes 18 or 19, old enough to take full responsibility for his actions and relize theres conseqenses.