Do not ask, "How are you?"
How do you think they are? You already know they've got something serious. Why are you asking? What are you going to do with the information? When they hear these words from you, what they will really be hearing is this: "Are you going to die soon?" If you don't already know the details of their illness, you probably don't need to know.
If you are looking for a conversation starter, instead say, "It's great to see you!" Or, "I've been thinking about you a lot lately."
If you mean it, go on and ask a much better question, "Is there anything I can do?" Then offer something specific. Say, "Could you use help driving your daughter to her lessons?" or "May I bring you dinner tonight?"
Do not offer unsolicited medical advice.
You may be burning to share your theory about which hospital is the best, which drugs or therapy saved Aunt Bea's life. But before you blurt it out, ask your friend's permission. For instance, you can say--in a non-leading way--"Are you happy with your hospital?" If they are, then button it up. If not, you can ask, "Would you like me to do some research on other possibilities, or offer you some suggestions?"
Do not theorize on why your friend created his or her illness.
Unless there's scientific evidence--and something your friend can do about it now--keep your theories to yourself. While we're at it, don't call the cancer a "gift" or a "lesson." If they think of it that way for themselves, great! But if you say it about them, it comes off as patronizing at best. Think about it: Did they need this great gift or lesson more than you or anybody else? And please do not say, "God must love you very much."
Do not pay a visit and expect to be entertained.
Make it clear that you are going to be bringing along a treat for the patient (and/or his or her family) to enjoy. Come prepared with something upbeat or interesting to talk about. Do not succumb to the temptation to use the immobile patient as your sounding board or therapist. Remember that you are coming to give, not to get.
Stay too long when your friend needs to rest.
Expect return phone calls. Instead, leave messages that say, "I want you to know that I'm thinking about you. No need to call me back. But if you need anything, let me know and I'll be right over!"
Don't bring meals in pans that will need to be returned.