It's possible it could be internal parasites, although I have seen discus in the past, just stop eating and waste away, while no other fish in the tank was affected. I guess one would never know for sure what causes this,unless the fish was autopsied upon it's death. It wouldn't be worth the cost, even if you find someone knowledgeable enough to perform it. Unfortunately, death is certain for this fish unless it starts eating, and it's not likely to happen.