3 Rules of Dating:
1. Never date a friend's ex
2. Never date someone your friend likes
3. Never date someone who likes your friend
I have broken two of these rules (with the same guy stupidly) and it didn't end well.
I advise to stay away from the girl if she used to date your best mate. My question is how long did they date? 1 week is okay 2 years is a no go area.