Damn, the people here are really harsh.
My advice is to just relax. Remind yourself that this isn't the whole world. You don't need to define what is going on between you, and it doesn't have to be serious. Be honest, talk to her about this, but don't let how she feels about your friend, or how you feel for her stop the two of you from having fun.
Don't plan too much, she doesn't seem to go well with plans, keep things simple. Don't think "I'm going to meet her and we'll do X at Y". If you're going to meet her try to expect only that, that you're going to see her, it might be with friends, it might be alone, you might do any number of things. Shouldn't matter too much.
I don't know how you feel or react in situations like that. Personally, if I love someone, I like being around that person, no matter what we do. If that person is happy, then I'm happy. If I'm part of that happiness, then all the better. If a person I'm in love with is in love with someone else, then sure, I'm going to be a bit jealous. Of course I am, I would be lying if I said I wasn't, but I'd still try to be with the person, and enjoy it. Whatever there is between that person and his/her love interest, I'm still going to love the person, I'm still going to enjoy their company, especially if that person is happy. And that person will have greater joy from my company if I enjoy it, and keep happy.
It's all about keeping a positive attitude, and not letting things that shouldn't matter get in the way. Don't let the future keep you from having fun in the present. Sure, she might get together with your friend sometime in the future, sure, she might not feel the same for you as you feel for her. Then again, she actually might, and she might not end up with your friend. At the moment the two of you seem to have fun together. Keep that up instead of worrying about possible futures.
Even if all of the cynics in this thread are right, and she is using you as a stand-by, what's wrong with that? You enjoy it, don't you? It's fun. Just don't take it too seriously, and try to keep a positive attitude about it.