Wow Anon what a wonderful question, thank you so much for asking.
Firstly, I have to say clearly you are a wonderful person for even considering this, and I am sure your friend or family member will understand your intentions even if they come out clumsily or not what you mean to say.
I know myself people say things to me that aren’t exactly what i want to hear, but I know when they mean it from a good place so I smile and thank them because they are trying their very best to be helpful and kind.
(For instance, I often hear “You’re looking really well!” which is meant as a nice thing, but it’s pretty meaningless when I feel like death. I hide my suffering, I’m not underweight and drawn, I wear make-up to disguise dark circles and pale cheeks. So it’s my own “fault”, but I do it because looking better does make me feel a little better)
Anyway to get back to your point, I’d say good things to say are “I’m sorry you’re feeling bad/worse/not so good just now, I hope you feel better soon”. “feel better” is a good phrase, because you’re not saying you hope they will be cured or fixed, but just that they feel better than they do now. Similar phrases like “I hope things improve for you soon”, or “I hope you’re able to get back to doing some of the things you enjoy soon”.
When you are chronically ill, being able to do the things you get a small pleasure from is really the one thing you cling on to on your dark days. Even saying “I hope you get a break from it”, or “I hope you’re able to get some good rest” are also understanding that when illness rules your life, all you want is 5 minutes from feeling it, or a good night’s sleep, or simply able to lie comfortably without pain or discomfort.
It shows how understanding you are that you get that we have a “normal for us” situation. Sometimes we have to accept that our “normal” or base level has changed, and that’s a very hard thing to accept. The past year or so i have had to realise that the amount of sleep I need has changed, and it’s tough. I feel guilty knowing I have slept the entire time my husband is out at work - a full 8 hours on top of some at night when he is asleep. But my husband makes it better because he says “You obviously need the sleep, listen to your body.”
Of course, the best thing anyone can ever say to someone who is ill that is ALWAYS welcomed is “Is there anything I can do for you?” and mean it.