A. If you like her, don't let it go to the "friend" level, once she's a friend you're always just friends. In my experience, dating friends never works out, if there's a breakup at that point every thing is seriously messed up. To avoid that, it's all or nothing date her or stay out of your boy's territory.
B. I've done what you've described, figured out that the girl was better off with me then the other guy, and each time, she left me (eventually) for another guy. Easy come, easy go.
Personally, I'd do it. I've done it before. Usually doesn't end well, so if you do get it to work, hold on loosely. Enjoy the time, but don't get too attached right away.