How can she be your best friend if she is now dating your ex?? I think you would be better off with neither of them in your life.

You should remove yourself from this situation. Their relationship is not your concern.

He wants you to help do what?
Elaborate.

Tell him you have to get the knife out of your back first.

Of course you refuse him, as he is on his own, and you want nothing to do with the situation.

I would shake his hand (without laughing) and wish him good luck (again without laughing). Then I would find me a new best friend, who understands you should never be desperate enough to date a friends ex's. Make sure you don't get involved or act like this relationship matters to you and when he comes back and wants to be friends because their relationship didn't work, just wish him luck and walk away.---good luck

Why do you think you have to do anything?
You tell him that is his problem not yours and stop talking to him.

Wish your best friend good luck. Move on from your ex and find a new best friend.