Health question!!!!!! (vision)?

so my dad has been having diabetes for about 20 to 25 years! he is 46 know and hes losing his vision! he lost his right eye completely due to diabetes :(, but we are trying to work on the left one. He can only see a little, but not that much! we are taking him to a really good eye doctor and he just had cataract surgery done yesterday (left eye), after this surgery he has another surgery will be done, the doctors will be taking blood out of his retina. My question is, will he be able to see better after blood is taken out of his retina?????, and can he be able to get a cornea transplant in order to see better? (the cornea transplant is something im just thinking about) i would really appreciate any type of information! thanks and god bless!