IF your Father in Law really did have good control of Diabetes
on lifestyle & diet control alone, he is unlikely to have developed diabetic retinopathy
. However, there could be other problems like branch retinal vein occlusion
or Age related macular degeneration
which could be causing this problem. However, if his retina is examined by a retina specialist, there should be absolutely no ambiguity to the nature of the problem. One thing is certain, he needs to be seen fairly urgently, as this damage could become worse.
Dr. Upsham Goel