Only with the friends explicit approval Yes it is.
It is never okay to date your friend's ex-boyfriend.
NO its not ok to date your sister ex boyfriend that nasty to date your sister ex boyfriend i would never date my sister ex boyfriend and i hope she wont date my ex boyfriend
Yes if you dont want to be friends anymore then do it unless he is ok with it then u can still be friends and then every thing is going to be just fine
Yes.
Date their friends.
Why not, if you remain friends.
The one mistake you made dating your best friends ex is not discussing it with them first to see if they would be OK with the idea. Best friends should last forever and should never let a guy/girl come between them. It's time for you to communicate in private to your best friend that you made a mistake by dating their ex and wanted to let them know and if it was OK by them. If not then stop dating the ex.
Friends
You can still be friends with your friend, but just try to remember to not get to close to your ex, and remember that your ex may be trying to make you jealous (it is possible that they aren't though). It is OK to be friends with an ex.
I would first ask your friend if it is okay with them, you do not want to tear a friendship apart.
Ask her if shes ok with it.
Maybe. My daughter married her ex's best friend. They both discovered her ex was a jerk.