It sounds like you have a classic case of the "Want What You Can't Haves." Now that he's with someone else, you want him again. Maybe he'll feel the same about you if YOU find someone else. Of course, if you do, maybe you won't care so much about what he thinks.
I agree that the best way to get over it is to meet someone new. She should be careful though that she doesn't just grab the first guy that comes along as a rebound thing.