My favorite thing to say to someone who is sick is anything unrelated to their illness. It's the same when a friend loses their parent or anyone close to them. Once I get past the obligatory "let me know if I can do anything" I want to be the friend they can talk about football, food, TV shows, whatever with. They have plenty of people to talk to about their illness or loss. They don't have many people that will let them leave that behind for an hour or two and think about something different.