How is Peanut's behavior? Does s/he go to the food and water station to eat/drink on his own? Are you still feeding him in your hand? I wonder if he'll ever be able to free-range if his vision is seriously impaired, but then lots of guineas don't get to free-range, so I don't see that as a reason to put him down as long as you have a nice, big run for him with companions. Just my uneducated opinion.