you should ask first. i would. no way i'm touching that without it being clarified that we're both okay with it.

No

As long as they tell you it's ok.

Nope. Even if they give you permission, because you'd be getting your friends sloppy seconds and if your friend is so eager for you to date them then there is probably something wrong with them. I would never date somebody that used to date one of my friends.

I would at least ask your friend 1st.

If you value the relationship more than the friendship.

Not best-friend but friend, and not his ex-bf....it was his ex-finance.