What should I do if my friend and I like the same guy?

First of all, I will tell you I've only seriously had a crush on 3 people; my ex-boyfriend (who I still talk to on a regular basis... We dated for 7 months and broke up because I had slot of family issues I was dealing with), a guy I've known since elementary school (who spent all of the homecoming dance doing the sprinkler with me freshman year, finally attempting to slow dance with me at the end), and this guy (ive known him since middle school and have gotten to know him really well these past few months). The guy who I currently really like is essentially the same person as me... We like the same movies (mostly m. Brooks, m. Python, and generally older films), tv shows, music (Frank Sinatra and Mumford and Sons), take the same classes, are both crappy speakers and are in love with band. He understands me better than anyone ever has, and we only started becoming really good friends 2 months ago when we were paired up for an English project... We couldn't stop talking after that. My friend has known him since middle school as well and was in the same science club as him and used to have a really big crush on him, though she went to a different highschool (up until now... The summer before our junior year.. She is now going to the same school As this guy and I). Well, today during marching practice she was reminded of jut how sweet he was, so she is going to ask him out tomorrow. What I worry about is the fact that she has yet to break up with her current boyfriend (though she is planning on it) and seems to go from one boy to the next... Like she needs to have a boyfriend at all times. She knows I like this guy too, but she also knows I've never really obsessed over boys, so she asked if I would care if she dated him, of course I said I didn't care... Which was dumb... I'm just afraid she's going to end up hurting either herself or him if they go out, because she is literally mortal enemies with every single one of her exes at the moment. What should I do?!?!?