I'm going to make this a running feature -- of course, this is all from my own peculiar perspective, so adapt as you will.
1) I love it when people offer to do specific things. A very sweet friend offered to bring over dinner. Another offered cat box cleaning, and another has made a standing offer for rides to doctor's appointments/tests etc. I haven't neede their help, but I might not have even considered asking for one of those things in the 'let me know if there is anything I can do for you' category-- and, if I didn't have those things handled I might not have even realized those things would have made my life easier until they suggested it. I really am fortunate to have the basics under control for now, as Hubby and Super-Mom are taking good care of me.
2) Ask about how we are doing -- and be ok with either details or a quick "things are fine" or "a bit tired, but otherwise coping". Sometimes we need a sympathetic ear and other times we don't want to get into the details -- we'll let you know which is which.
3) Don't call me a hero for living. I'm doing chemo, I have no hair and one boob. All but the boob will be resolved sooner or later -- I'm not a hero, at least not yet. If things get much, much, much worse and I live to be 109, then you can dub me a hero. I'm not a hero for working during chemo -- most breast cancer patients work nearly their normal schedule. I'm just doing my thing and dealing with the inconvenient stuff -- you do your thing and have challenges too, and neither of us are heroes.