However I'd also say that if the not following the doctors advice causes long term damage and affects quality of life I'd ask is it worth it?
I say this as someone who has multiple chronic health conditions. Also I have a friend who went blind after eating things they shouldn't have over along period of time and now regrets it.
You present this like it's a simple, black and white issue. It is not though.
I'm going to make a wild assumption that your blind friend is diabetic. There seems to be an element of blaming your friend for their blindness, that really isn't fair or reasonable.
Do you know how common eating disorders or disordered eating is in diabetes? As in, a real actual mental health condition? About 25% of women and girls fit criteria for anorexia, bulimia or diabulimia.
Behaviours of disordered eating become entrenched in a clinic where you are taught to fixate on numbers and restrict food. All the things you work actively to discourage when treating non diabetic patients with eating disorders. But numbers and restricting have to, to a certain extent, become your obsessions when you have diabetes. So it is not shocking when things go tits up.
Then there are all of the folk without disordered eating, but with depression from diabetes distress.
Things may be improving slowly now but diabetic clinics used to heavily rely on shame and guilt as techniques to try to bring young diabetics into line - tools which don't work, and not surprisingly, compound the problems and make it all far worse.
Additionally - you talk about "following the doctor's advice" like it is a simple fix it and all will be well as long as you do this faithfully. That simply isn't true for diabetes. It is an incredibly minimised condition where a majority of doctors remain reluctant to fully address all the variables which affect blood sugars.