I'm really sorry to hear this. My father lost his eyesight around 15 years ago due to diabetes so I've been in the same exact position and know how you feel.
It's too late for my dad to help his vision because he wouldn't give up his SAD in time before the damage was done. He was in denial that the foods he was eating in excess (fried/junk foods, sweets, etc) was killing him.
Yesterday he went to get a checkup and the doctor told him that he is no longer diabetic and he is in perfect health and they thought it was due to exercise but he doesn't work out at all. It was due to my mom making him eat healthier over the years. Diet trumps everything. He's not vegan but I guess you can say he eats a majority vegetarian, mediterranean diet (since we're from that region and that's the food my mom knows how to cook). My family eats meat maybe 2-5 times a month and a moderate amount of cheese and eggs. They've really cut down on meat since I transitioned to vegan and influenced them, I guess.
He eats a lot of oatmeal, bread, oil, eggs, cheese and beans and follows the "eat until you're full, save the rest for later" rule religiously. He's always talking about how oatmeal is his saving grace because there's a lot of fiber in it so it helps you feel full for less calories. That really helped him manage diabetes lately since he monitors his blood sugar and knows exactly how much he needs now.
He's almost 60 and expressed interest in going vegan for ethical reasons after I had conversations with him about factory farming and animal cruelty which is kind of cool because he used to be an avid fisher and would eat like 50 chicken wings in one sitting. Going blind has made him oddly spiritual because he doesn't have much going on right now due to being forced into early retirement so his situation is rough but he still remains positive. His trade was entirely visual based and a huge percentage of blind people are unemployed right now, unfortunately.
So to sum it up, a lot of what has helped him is portion size-- eating for the right blood sugar numbers. Plead to your parents to take care of themselves. Do anything to get them on board. This whole experience really affected our family and my childhood. It was a huge life lesson and I am pretty health conscious because of it. I hope this helps.