When someone has a chronic illness, it's easy to try and give well meaning advice. I'm guilty of it. Sometimes I was trying to be helpful and sympathetic, sometimes I just wasn't thinking and I've stuck my foot in my mouth.
But, I have a bit more sympathy for those with illness now. I hope that it's teaching me a lesson about having some consideration and trusting others to be in charge of their own health. I'm not a doctor or health professional, so something I may have heard or read is not necessarily going to be of use. The person is probably well aware of what they are doing and have made all their health decisions with their eyes wide open.
I have been suffering from chronic fatigue. Not CFS, just chronic fatigue as a symptom. I have been given all sorts of well meaning advice from resting more to exercising more and so on. Finally through the proper method, my doctor and I have worked out my problem is to do with sleep. Not the amount of sleep, but the quality. Thankfully I'm now under a specialist and having a sleep study done soon.
When we give advice to others, especially when they are sick - it can sound like criticism. Sometimes it's probably best (at least for me anyway) to lend a caring ear and keep my mouth closed, unless I have been there myself.