Most Helpful Guy
You broke up. Anything that he does after that is irrelevant to you and vice versa. If someone wants to try to mend things now, you either are ok with that or not... and that makes the decision for you easy.
Most Helpful Girl
Did you leave him? If so, it's just a rebound.
Did he leave you? If so, he wanted her all along, but didn't want to "cheat".
I had a guy for that to me once. He wanted me, but he was in a relationship. He left his girlfriend, which I didn't even know about, dated me long enough to get it, then was back with her literally the next day.