I say let them. If it works out, you'll have helped them find happiness, and you won't have been seen as petty, or jealous. If it doesn't work out, you can tell your friend "There's a reason he was my ex first."

I don't see why this is a problem. You aren't dating him anymore.

Because it's weird... there's so many people on this planet.

Probably because a lot of the time, when people break up, they don't want anything to do with the person they broke up with. If this guy is now dating the person's best friend, who they presumably spend a lot of time with, they'll be required to keep some degree of contact with this guy.

You and your bestie will have something new in common

I hope you mean EX-boyfriend.

Wow, my wish game is strong! Ok, now that I got my way, what seems to be the problem?

Well, that’s too bad for you, it’s their lives not yours. Find a way to deal with it or find new friends.

Since she's your best friend, you two need to have a chat. You need to be open-minded about her potential feelings, and she needs to respect that you share history with this man. I hope you can find happiness, but you also need to remember people aren't robots and cant always control who they fall for.

your friend is a bad person

It sucks , but he is your ex , nothing to do with you ,

That’s rough, but they can’t control their feelings. Hopefully it’s just the “forbidden fruit” effect and it doesn’t last long. Prepare for the worst and hope for the best!

Let them
When it goes down the drain and someone cheats your friend will likely come crawling back
Then you can laugh at her

Dont date at work.. That's more or less inbreeding, it never ends well.

You clearly don't understand the meaning of the word "inbreeding."