This story is wrong somehow. It has to be fake. Fryer grease potatoes and bread would not make someone go blind. Obviously a kid who could maintain this diet for so long has to have other issues as well. But lack of vitamins would not cause blindness in a teenager.
Vitamin A deficiency can.
What Is Vitamin A Deficiency?
https://www.aao.org/eye-health/diseases/vitamin-deficiency
Vitamin A plays an important role in your vision. To see the full spectrum of light, your eye needs to produce certain pigments for your retina to work properly. Vitamin A deficiency stops the production of these pigments, leading to night blindness. Your eye also needs vitamin A to nourish other parts of your eye, including the cornea. Without enough vitamin A, your eyes cannot produce enough moisture to keep them properly lubricated.
Vitamin A deficiency is the leading cause of preventable blindness in children worldwide. An estimated 250,000 to 500,000 children become blind every year because of vitamin A deficiency. Half of these children die within a year of losing their sight.
In pregnant women, vitamin A deficiency causes night blindness and may contribute to maternal mortality. Vitamin A deficiency also harms the immune system (the body’s ability to fight disease). This increases the chance of death from malaria, measles and diarrhea.
He had to have other health conditions. My guess would be type 1 diabetes and the high carb diet and sustained high blood sugar destroyed his nerves.
Apparently it can. See: Nutritional optic neuropathy and Toxic and nutritional optic neuropathy.
It looks like the blindness is caused by a lack of folic acid and vitamin B-complex vitamins, specifically, thiamine.
The article goes on to say that the teen was not completely blind but had a blind spot in the middle of his vision, which means he could not drive. He was able to walk around and function because he still had his peripheral vision.