I was once told that "All's fair in love and war", which sounds very harsh. I have heard from many circles that it is a no-no to EVER date your friend's exes, so I would also wonder when they really did get it together.
It depends.
If you chucked him, then you probably wouldn't be bothered. If he chucked you and you were devastated, then it would be a horrible thing for a so called best pal to do!
I think the unsaid rule of thumb is to by pass your friends ex's if possible but as a matter of etiquette the friend should be asked for her blessing!
Never appropriate. I have a friend who is 60 and split with her hubby a year ago. The best friend now ex-BF- is now with him. My friend is more upset at losing her friend than the husband. I told her that even my teenage students know this rule! If one dares break it there will be fight for sure!!!
I wouln't want to be with any husbands of my friends. EEEWWW!! I know all the bad stuff, so why would I do that?