some things are off limits. you'll get over it.
my best friend told me all her girlfriends would talk to her brother and when they're relationship ended, they stop talking to my girlfriend. it causes too much conflict.

Are you sure it's off-limits, though? If he's really your best friend, wouldn't he rather you date her than someone he doesn't like or doesn't know? Have you talked to the girl or your friend?

If you don't think that it would ever work out, then you need to focus on other things and try not to be around her for a while.

Good luck.