I love the suggestions of what *is* Ok to say. Because so often people just...have no idea. They don't know if it's OK to talk about trivial, silly things when someone's going through something so major. They don't know if talking about the future is OK, because the person might not be there to experience it. They don't know if talking about similar experiences would be helpful, or sympathy-jacking. It's not an excuse for saying ridiculously insensitive things. But I think sometimes people genuinely can't figure out what is and isn't OK to talk about. And, for certain conversations, you might have some people who find something comforting that another would find dismissive.
My mother died of a terminal illness when I was 14, and I never heard so many insensitive comments while she was sick and after she died. People said ridiculous, appalling things. And these were not people who were jerks or trying to hurt me in most cases. But I think sometimes people have no idea what's OK to talk about. and I always hated having to deal with someone *else's* grief, or confusion.
So, yes, I'd absolutely say something ahead of time like 'You don't have to pretend that she's not sick, but Amanda would really rather talk about what the Lannisters got up to the season finale, than spend more time talking about her illness.' Because people may be afraid that talking about trivial matters would be insensitive.