A question that I get all the time, especially from my younger college-age readers is, "Marius, is it okay for me to date my friend's ex?"
My simple answer is "NO!"
Have some common sense and think about it. Even if your friend tells you it's okay and gives you his blessing, every time he sees you with her, he'll probably feel weird. And every time he sees you kiss her or get touchy-touchy with her, he's probably going to feel like a stake is being driven into his heart.
It does not matter if he dumped her or is over her. Jealousy is a powerful emotion and seeing his best friend with his ex is NOT going to feel good. Think about it...
How would YOU feel if YOUR friend started going out with YOUR ex? Better yet...
How would YOU feel if YOUR friend started going out with an ex of yours that you are STILL attracted to?
Remember what I always say: Women come, women go. But friends are for life!