What do you think about me trying to hook up with my ex's best friend?
What is your reason for trying to hook up with your exes best friend?
It will prove how cool I am. It will also make him jealous.
Hooking up with your ex's friend just to make them jealous is pretty messed up. Also, hooking up with someone for a reason aside from liking them doesnt make a lot of sense. You are better off trying to attract someone outside of that social circle completely.
What, so I shouldn't hookup with her then?
Hooking up with your ex's best friend will make you look like a jerk. It's wrong to do this and shows that you are simply wanting your ex's attention.