I'm probably one of the dumber people in life who has actually gone after a friend's ex knowing full well that my friend (girl) wasn't entirely over the guy she broke up with. I know, HUGE drama! When the guy and I got together a couple of weeks after the break-up, my friend refused to speak to me. She was very adament in her decision and kept up the silent treatment for several more weeks. At one point, she even sent one of her friends to confront me (how junior high is that?) and her friend and me ended up in a fight (well, maybe fight is too strong a word. Words were exchanged, slapping, hair pulling...no punches. I'll just call it a cat fight. It was so stupid really, we were in school and everything, an art teacher broke it up). Anyway, the guy and I dated for ten months and when all was said and done, the guy ended up cheating on me and I lost two of my best friends.
Now, I NEVER go after a friend's ex. You may think he's hot now, but what happens when the relationship ends? You definately can't go back to being friends with the person you screwed over. Unfortunately, it just doesn't work that way. Remember this: in a boyfriend/girlfriend relationship, nothing is stable. You have ups and downs and sideways and diagonal ways and sometimes, fight it all you want, the relationship fizzles and falls apart. But friends, they were there before the boy and they'll be there after (as long as it's not one of their exes). You're better off going with neutral territory.