Is it ok to date a girl who has just been dumped by your best friend?

Yes, but I'd take it slow and easy because she's upset and probably grieving over the break-up. Yes, I said "grieving" because that's what people do when someone loves another and they don't return that love. If you ask her out and she agrees be prepared that she will want to talk about her ex (and your friend.) Have patience! If you aren't prepared to go through this then don't ask her out. Good luck Marcy