[84LX89GT]

If i were you i wouldn't go out with her. I was in that situation a few years ago and the girl wanted to go out with me after they broke up. Here are my reasons for not doing anything:
1. If your friend is very close to you, and you've been friends a long time, the friendship is something that you can't easily replace.
2. Even if you don't go out with the girl you can still be great friends and hang out together as a group....going out with her the chances are things will get weird, NO chance of that if you don't
3. Wait a few months and talk it over with your buddy, make sure he's absolutely over her and that he REALLY doesn't mind you going out with her.
4. As with above waiting awhile for them to resolve any feelings for each other helps make sure you don't get in a "rebound" relationship with her and ruin your friendship with him while you're at it.
5. If you're friends with both of them, chances are they won't dissappear in a few months, so rushing into something doesn't make much sense. Although it's hard, waiting for things to settle down and be completely resolved is in my opinion the best way to go.

These are the decisions in your life that you remember 20 years from now, think about what you REALLY want from each of them and decide what you want to do.
Good luck, and i feel for your situation.