You absolutely could.
But should you?... Probably not if you value your friendship.
Can you date your ex best friend's (Still friends but not best friends) ex boyfriend when they broke up about 6 months ago? I had a crush on him when we first met but helped my friend and him get together. She has a new boyfriend.
I have to agree with Soul Fly on that. It really depends on what kind of person your friend is. If she's really strict and hard, I wouldn't do it. But if you two are still very close and if she's a true friend who will understand you but will be disappointed then I think it's fine. I would suggest the person you not only think but also feel that the friendship/relationship will last.
I would tread carefully. Some of what went wrong with their relationship may still be fresh--it was only six months ago. If the two of you are very close friends test the waters with her. If she seems okay with it, ask her directly. If she says no, than no don't date him if you value your friendship.