Generally they won't have the attention span to get all, or even most, of the facts. But there's no need to get offensive, either.
I just agree and wish them a nice day. If there's any easy advice that might help them, I'll pass it on before they go. It's the least I can do.
What they don't know won't hurt them...; until it does. Until then, they have no good reason to live in fear. Once they do know, they'll probably thank you for not filling their days with moments of unnecessary terror; if they give it some real thought. My own exploits brought me through Non-Hodgkin's Stage 1A, and then, four years later, Hodgkin's Stage 2B. Been in remission since 1997.
Cancer sucks enough all by itself without having to live one's healthy days with anticipatory fear. If I had known what was coming, I'd have lived my remaining life in a dark cloud. Nobody deserves that. When I did find out, I lost my sense of reason for about a week; good thing I never acted on about half of the things that were galloping through my mind around then.
Even when we're circling the drain, we can still give others some moral support. We should.
Greg