I think that the airline were not unreasonable in their attitude. I think its very easy to become sucked in to an emotive topic and see it from the eyes of a tired mother with a SN child. However, you need to also see it from the view of an air line who are responsible for the safety of 100's of passengers, including the teenager concerned.
From the airlines point of view
Mother in economy asks for a hot meal for her child, a request that happens a lot. You explain that she can't have one because hot meals only in first class. Mother starts saying shes a platinum card holder so you have to give her a meal. She says that her daughter has Autism and needs a hot meal. You explain that she can't have one . The reason for this is due to a limited number of hot meals on board the but the Mother is becoming irate thus making you unable to put your point across correctly.
You offer a hot chicken sandwich. This isn't right. Meanwhile other passengers are requiring your attention.
Mother is becoming more angry. Then starts spouting off about her daughter scratching & having a meltdown. You don't know what extent this scratching/meltdown is and to whom it will be. Regardless of any SN you need to asses the danger to other passengers. If the teenager becomes irate then cabin crew wont be able to restrain, someone gets hurt.
Inform the captain - as will be required. The captain decides they can't take the risk. Captain radios through to ground control. Plane diverted.
In the meantime you have a left over hot meal - give it to teenager to help prevent melt down.
If the mother hadn't mentioned the 'meltdown' or 'scratching' and had been less 'mouthy' perhaps the cabin crew would have been able to have had a proper conversation with her rather than causing such issues.
You cannot act in such an aggressive manner on planes.
FWIW I have food requirements which I informed an airline of in advance. I was given a standard meal - despite repeatedly saying are you sure this is xyz free and being assured it was. I ended up be horrendously ill. My other half was fuming, but he remained calm because acting aggressively on planes will get you kicked off - and quite rightly so.