Don't insist on knowing their prognosis. Don't be afraid of them. Cancer isn't contagious. Some people lean way back when they talk to cancer patients as if they're afraid to catch it. Don't stop writing, visiting or calling.
Cancer takes a long time to treat.
Stick with them for the long haul. The burden is on you, the healthy person, to stay in touch. The best thing you can do is treat them the way you always have. Ask them what they need, what they'd like to eat, if they'd like to get out to the library, the museum, or to see a movie. They are still your friend, sibling, aunt, uncle, co-worker, spouse, child, parent.