That breaks my heart.
My Lady has been a diabetic for 6&1/2 years now so I know a lot about the disease. Maintaining a diabetic dog is a big commitment in time and money, but it is a completely managable disease if you do.
Blindness is one of the main side effects in dogs, but unmanaged diabetes also damages other organs you can't see like the kidneys. My Lady still has her eyesight and has had no other organ damage after all these years. I treat her as I would a human diabetic, though, and test her blood sugar myself daily.
Unfortunately, most diabetic dogs do go blind, most within the first nine months after diagnosis, usually because people wait too long to take them to the vet to be diagnosed or are too slow in getting the disease regulated. The good news is that dogs adjust very well to blindness because their sense of smell is so acute.
I hope little Maggie finds a special home with people who will take good care of her from now on.