Hmmm... Guy and ex had been close friends before they got into a romantic relationship early on in high school. He was about two years her junior but they got along fine. Guy had been a sweet albeit nebbish little dweeb in his mid-teens although the girl had been able to look past that for the most part. They had a falling apart though when the girl gets pressured by her other friends into dating older more attractive guys (with the guy somewhere between an awkward puberty phase) and that was sort-of still the case until she had to move away to College. The guy was royally messed up by this and, coupled by the fact that he was alternately beaten black-and-blue by the puberty and hotness sticks, he ends up being a magnificent bastard.
Invited to the wedding, the guy plans on having his way with the girl but ends up realizing that she's about to marry an abusive rich asshole who knocked her up. Maybe in the end, the two go on to humiliate the new lover, eventually helping the girl finally stand up for what she wants to do with her life. In the end, she might ask to start up a relationship with the guy again but he spurns her, picking up a couple of bridesmaids as they part ways. Well, just a thought.