If he had a medical card , then they would not be entitled to ask for the money back.
If he had no medical card and no medical insurance then, they are entitled to their money. I dont think it works the way it used to work years ago - that if you die all your bills die with you.
I assume that there is no assets left in this name ? so there would be no way for them to get the money back. In fairness though if they treated him well and looked after him while he was ill then some sort of contribution should be made to pay off part of the bill even if it is only a small sum.