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.

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

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.

And you should let them. I married my best friends ex boyfriend. Just because he’s your ex doesn’t mean you should be upset about him finding happiness. You should worry about yourself and try to do the same