Once my husband was working in a church building and said he couldn't do something because of his allergies. The man replied, "If you didn't lack faith or have unconfessed sin you'd be healed."
A few minutes later my husband asked the man to hand him his glasses. Taking them, he put them in his pocket, and started to walk away.
"Hey! What are you doing? I need those!"
"No you don't. If you didn't lack faith or have unconfessed sin you'd be healed."
The man got the point, apologized, and received his glasses back. As far as I know, he never made that comment to anyone else.
People often give unasked and unwanted advice to others with invisible disabilities. Some even give it to those who have obvious ones, though it's less likely for someone with an amputated limb to be told acupuncture, herbal remedies, or prayer would correct their condition.
Although it's usually done with good motives, giving unrequested advice is simply rude, especially to someone who is not a family member or close friend, even if it might really be helpful. It's no different from telling someone they would look better with a different hair style or that they should get rid of their car and buy another kind.
And, since anyone with a chronic condition has probably already considered and tried everything others can suggest, it probably wouldn't help them anyway.