Most Helpful Guy
It's likely that he was flirting with, or at least considering, this person before he broke up with you. She could just be his rebound though, but it's not worth the effort of finding out really. You should take it as a cue for yourself to move on too and not pay him any more attention.
Most Helpful Girl
He broke up with you cause chances are he was talking to or had interest in the other person during your time together. Anybody who can move on that fast and start flirting with someone or date, is someone who wasn't emotionally invested in the person they were with from the get go. They were just settling and waited for someone they would really like to come walking by, in my opinion your ex is just another one of these D-bags who did just that. You don't need him and he honestly did you favor so you can now find someone who will ACTUALLY want to be with you and not someone else. And if things should go in the crapper with the other girl and he tries to come crawling back, tell him he got the wrong girl and the wrong address and point him back to the door.