I don't know what you can say specifically - has she confided in you about the nature of her illness? But the main thing is to just carry on being her friend, just as you were before she was ill, and be there for her if she needs to talk.
Sorry I can't offer something more useful - I'm sure others will though!
EDIT: Just had a thought - I know it's difficult when someone is (seriously) ill, but if you can I'm sure she'd appreciate it if you didn't tiptoe around her, and just act normally. I've heard people who have had serious illnesses say that it's really a pain when friends/family suddenly behave differently towards them because of the illness. Of course it's pretty difficult not to behave differently, but I'm sure she'll appreciate just chatting about everyday stuff with you, and feeling as if at least some part of her life is "normal". Hope that makes some sort of sense!