People who dare say such a thing to someone ought to be given the right and left uppercuts of mind-your-own-business-ship.
Penny, ask this person: What makes you sick? What physically, biologically happens in your body to make you sick? Of course, they'll probably give you a spiritualized answer to that question. Tell them, "You're dodging the question I asked. What makes you sick?" Refuse to accept any answer unless it is along the lines of infection, germs, viruses, genetics, conduct (smoking, overeating, etc.). Ask them to then explain that answer in the best medical terms they can. Just let them talk it out -- silently let them finish their answer and wait about 5 seconds after they're through talking before saying anything.
Then, ask them: Alright, what if you could stop that from happening -- would you do it? If they try to give you some answer on HOW they'd do it, tell them again -- "you're dodging the question". Repeat it: If you could stop that from happening would you do it? (Only an idiot would say no. If they do say no, then end the conversation with, "Well, you are an idiot.") When they say yes they would stop it if they could, tell them: "That's what I'm working on. And until you get a medical degree, you need to shut your mouth. I trust God. But you know what? He trusts ME! He gave me my son, you didn't. He put me in charge of his care, you didn't. So, I don't really care if you think I'm doing right. You're out of bounds and you need to step back and close your mouth. If you can't pray for me and my son to be well and for the doctors to help him, then do me a favor and forget about us."
That's my recommendation.
Human government bears the same relation to hell as the church bears to heaven. (David Lipscomb, On Civil Government, 72).