I've failed to complete this post a number of times. It's laden with caveats and nuances and limited to just my learning, but it's important that everyone becomes familiar and adept at how to support the cancer patient. But I think it's applicable to any patient whose life is threatened.
Thanks to science and medicine, many Americans get well into middle age before a friend becomes gravely ill. We know how to talk about in hushed tones amongst ourselves but barring previous misfortune have no idea how to properly respond. But even more importantly I found many adults flounder to respond because they haven't had to grapple with overwhelming senses of despair and helplessness and the secret they dare not verbalize that they and their family are just as vulnerable. The last thing a patient needs is for their friends and acquaintances to respond from a crazed, fearful place. For some it's just too much and they can't even show up. For others it's becomes about them, offering to help just to avoid facing what I describe above. Plenty of others want to help simply because they don't know what to do. Some people figure it out quickly and get with the program. A small few have the life experience to approach it the right way, while some other expert humans are simply so comfortable accepting that life is change and emotions are life that they're naturals.
So, the first part of being good to the patient is to get your own head together. Be mindful of how this is making you feel. Channel your fear of this happening to your family into a celebration of being alive. Accept that feeling helpless isn't a shortcoming. Recognize despair and grief are part of the pantheon of life experiences. You do this because you don't want to make any of this about you. Support and love must flow to the patient and fear and discomfort must flow away. If you are not good at expressing love and gratefulness to your friend, learn how to do so quickly.