Once I was in line for the initial BP check at the security checkpoint at IAH terminal C and saw a disturbance. It was a mother and father with an approximately 18 yr old autistic boy who was becoming very upset that his mother was leaving (she had already been directed into the security line). They tried discussing it with him quietly but he gradually was becoming more and more upset. After a couple minutes he went violent and hit his mother in the face, knocking her glasses off her face. Fortunately, police had just arrived on the scene and the father and police had to take the boy down. The boy was a good 6 feet tall and was flailing and punching anyone near him. It took the father and two police to restrain him on the ground. Everyone else in line had to quickly move back to avoid the melee.
I would just have to think that the family knows better than anyone how this particular individual might react in certain circumstances. If they perceive any reasonable risk that they might respond violently, they should not be brought to airports or put on airplanes. I know this is a very difficult situation for the family but they have to respect others safety as well.
I would say in this situation, the key is to whether the mother did state there was a threat of a violent reaction. If she did, the pilot made the correct decision and the family may be liable based on whether they felt there was a reasonable risk she could react violently. It sounds like the flight crew went above and beyond their responsibilities to provide assistance to the child and alleviate the situation.
It sounds like the mother complained that the pilot did not take a second assessment after food was provided and that should be considered as well.