This seems like a really tricky situation. I wonder if the girl had been younger things would have been handled differently? Would a pilot view a 6 year old's melt down as a potential threat? Maybe the flight attendant could have tried to be more helpful when the situation first arose, but it also sounds like the mother might have escalated things. One question I have, is why they weren't prepared with food/snacks for their daughter? If they know that one of her triggers for a meltdown is being hungry AND that she is a picky eater, why would you not come stocked up on the stuff she normally likes to eat??? If she had already calmed down I think making an emergency landing was a bit over-kill, but I don't know the protocol - maybe once the decision is made they have to follow through?
I am reserving full judgement until there is more information. Initially, it does sound like there was no longer any issue at the time the plane was stopped and the family asked to leave, but it also seems the mother was a bit threatening prior to that when a melt down was imminent (the, she'll scratch people comment quoted above--which may have been more than that really, as we are only hearing mom's version of events) and once the decision was made to divert, it might have been hard to take back, you know?
My gut reaction is that neither the parents (and their daughter) did anything wrong in trying to fly, asking to buy a hot meal, etc, but also that the pilot was not wrong to divert when told there was a threat of a 15 year old passenger harming someone and then deciding to disembark her and her family when they landed even if she were calm then (if she had stayed on and melted down after that and scratched someone's eyes, etc then suddenly the airline is liable for having knowing allowed the threat to remain on board). It is just one of the sucky situations that can happen sometimes in which no one is to blame. (trying to blame the airline as the family is now doing, on the other hand, seems like it might be wrong, based on what little information there is so far)
As to the food: the article I read this morning said that the girl will not eat room temperature food. They had snacks but she refused to eat them because they were not hot. Apparently they also ate at a restaurant in the airport before the flight but the girl also refused food there (even though it was hot), which led to her being hungry and melting down in flight.
I was left wondering how mom was so sure that a meal from First Class would solve the issue, when the girl refused a hot meal just prior to the flight? I definitely feel for the parents, who seem to have been trying really hard, but on the other hand, I am not sure you can count on hot food on a domestic flight at all these days (they were lucky something was available in first class to feed the girl--not all domestics provide a hot meal there, and/or there might not have been any extra available to purchase).
It is so hard to balance things and second guess what to do and how to handle stuff with an autistic child or teen, but it almost seems like once the girl refused to eat at the airport, if they knew she would turn down room temperature snacks and that she was likely to melt down if too hungry, it may have been smarter to talk to the agents on the ground and rebook the girl and one parent for a later or next day flight when she could board already well fed. Stinks to have to do that and possibly expensive, but also seems like better than counting on an airline being bale to produce a hot meal that the girl would not snub on demand. Better for both the girl and for the other passengers and crew.
