Is this ok?

Noooo! Don't do that. That's her best friend I know you broke up with your ex girlfriend but your ex has feelings too. Ask yourself if my ex started going out with one of my friends how would that make me feel. You wouldn't be happy would you? And just because you broke up with her doesn't mean she still doesn't have feelings for you. Why did you brake up with her?

Yeah there's no issue here. She might have an issue dating her best friend's ex though.

nope, you will absoloutly ruin a friendship and that's not okay. you will regret it and she will just become annoying and clingy because she won't have her best friend to rely on and you'll get annoyed and they both won't thank you for it. strongly not advised

good luck :))

no.