aww man.. that's just so sad. But you have to keep yourself together. If she doesn't come back, you are eventually going to have to deal with it, and it would be a shame if it messed up your life, so young. Watching somebody slowly suffer and die (for 3-4 years?), gradually getting worse.. that has got to be the most horrible experience. And while of course you should be there for her, don't forget to do stuff for you too; don't let yourself die too. 'cos man, the truth is, you're might live till you're 86 and she's only going to get to, like 22 or 23, if the doctors are right?
A friend of my brother's died of a rare form of cancer, and it was so damn sad, even though I didn't know him. I know this depends on the person, but his reason for clinging to life was that so many people had worked so hard to keep him alive, it would be a waste if he didn't. He drove too fast and did everything (and more) than his body would let him, because he wanted to pack a full life into 21 years.
I don't know why i'm saying this- I guess I think it's important that your friend doesn't just lie there dying, that she gets to experience some things, and if you can help her with that it would be the most beautiful gift you could give her.
*sniffle* .. man this is all so sad can't imagine being in that situation. So, as I said, take care of yourself- you wont be any use to her if you're sad all the time. I hope the surgery goes well.