Personally I think it is NEVER ever a good idea to date a friend's ex. I suppose however you have to look at the facts and circumstances of each situation differently. How good of a friend is she to you? If we are talking about your best friend, someone you have been close to for years, who has been there for you whenever you needed her, then I definately would not ever jeopordize losing a friendhip as valuable and as precious as this. If however she is more of an aqualintance, you belong to a group of friends that hang out together and you and she just happen to be two in the smae group but do not have a srong independent bond, then that may make things different.
You also have to look at the seriousness of the relattionship bewtween you friend and her ex. Was he the love of her life? How long did they date? If they date exclusively for 3 years and she is devistated by the breakup, forget him and keep your freind and move on. If however she is one of these grils who has dated every single man in your area and her longest relationship has been 4 months and she claims to love every single man she dates and every man happens to be "the one...." give it some time and soon you my find that she has moved on and is madly in love with another man an may even give you her blessing.
There are two situations in which I have dated a friend's ex/ (1) my very best friend from grade school on up 20 + years of friendship was dating a man, but he made it clear to her that he only wanted to be friends from the beginning, he was not interested in a serious relationship and he never referred to her as a girlfriend. When they were together I lived in another state, they broke up and when I moved back home she had laready dated and broken up with and been heartbroken over 3 other men. She was finally engaged. I explained to her that we lived in a small town and that if I was not permitted to date any of her old flings then I could never date, we laughed and all was ok.
Another freind, who I always knew was more of a fair weathered freind than i was willing to admit, dated (if you could call it dating, they had three dates, but did sleep together) my best friend-I set them up. She was dating other men at the tiem as was he. it did not work out, she later married another. Aftr I divorced my best freind (male) and I eventually acted on our feelings we had always had but never admitted to. We have been together for 3 years now