I just found out one of my best friends has leukemia...?

One of my closest friends had cancer to but sadly he lost his battle. Just be there for her, that's the best thing. If she wants to talk about it with you then do that and visit her when you can and maybe bring things that you think will help her feel a bit better. Try not to be different about her and remember her illness doesn't make her a different person than she's always been.