Funny, my two best relationships ever, BOTH of these girls had a FUKKED up best friend.
First one bordered on mental instability. I think it had more to do with her being jealous of my girl having a good relationship and the fact that I "took her away" from their friendship. A friend of mine went on a date or two with the friend, I think her name was Esmerelda, and he got his own taste of the wicked witch, so I know it wasn't just me!
The other one was a mature 21 yr old with an IMMATURE 18 yr old friend. Can you imagine having to hang out with an immature 18 yr old? Yea, it was that bad.
To answer the question......I would take note of a woman's friends, but I wouldn't let it be a deal breaker.
A chick I dated last year has a, lets just say "promiscuous" best friend. I actually like the girl and all, got along with her great, but with a friend who sleeps around/abuses drugs/treats men like sh!t/etc. is always going to have at least SOME influence on your girl. I even remember when we were first hooking up, she was in another city visiting the best friend about 2 hours away from me, and she calls me up kinda freaking out because her friend had left her with some random dudes that night. So yea, I wouldn't disregard it altogether.