I love that you are asking this! You must be a great friend.
I would second everything already listed. I think a card with a heart-felt note goes a long, long way. It just lets the person know that you realize they are in a tough spot and you love them. I also don't think it hurts to send a note to the sick person even if you aren't close. You can say something like, "Hi. I am a friend of your daughter So-and-So. Although I don't know you personally, I am praying for you during your illness."
A gas gift card can be very helpful. When I've had family in the hospital or needing many doctor visits, they commented on how fast their gas tanks seemed to go to empty!
The following article really helped me when a very close friend had cancer. I don't agree 100% with her comment about prayer, but I completely respect her position. I thought it might help you or other GC readers.
http://www.theawl.com/2011/06/actually-awesome-things-to-say-to-a-cancer-patient