Your support is going to be so important to her. I'd say moral support, love, encouragement, etc. is going to be appreciated and so appreciated. Also, though, some "practical" support would be invaluable, I'm sure. Maybe some casseroles, or other easy meals for her family, stuff that can be kept frozen, then re-heated easily. Or some help cleaning up around the house. If you don't have lots of time, maybe help organize a schedule with other friends, so that everyone can pitch in a little bit of time and help, that way she still gets lots of help. Even though it sounds like she has a nice, big family, they might all appreciate some help, especially when she's most incapacitated, so they focus a lot of extra attention on her, and not so much on day-to-day stuff. It'd mean a lot, I'll bet. (I had a fairly long hospitalization after my son was born, with a really bad infection, and having someone cook for my family was so appreciated!! Also, visits....I wished I'd had more visits from friends, even. You don't have to do or say anything "right", just being there is great.)
Having emotional support is definetly going to be key, though, I think. Just having a shoulder to cry on, someone who will just listen to all her fears and everything she is feeling. The treatments and surgery can be so emotionally and physically draining. Helping her stay up-beat and strong, and inspired, but also allowing her to feel all the hard stuff, that's just going to make all the difference, I think. Bringing bright flowers, balloons, lots of supportive cards, etc.