Should I allow one of my best friends to ask out a guy I like?

I have a friend who just told me today that she was planning on asking a guy to the Sadie's dance that I had planned on asking. And then she asked for help on how to ask him. Of course I was totally shocked, but I helped anyway. Then she asked me if I was going to ask anyone, because I had said that the day before. And so I told her that I had planned on asking the same guy and now she feels horrible. My automatic response is to say No, no, it's fine, I'm not mad or anything, but I didn't know how to say anything else. I'm not mad, but it's not like I can ask him now. Please help me on this!