VMK said:
Hmm... Sending hate mail is hardly the right way to act in such situation. Your very first action should be talking to the parents of this child and explaining your position. If they manage to successfully calm him down, then problem solved.
If not, however, then she should have filed an official complaint, backed by petition of other people, living in this zone (sorry, I don't know how things work in USA, but you get the idea). Then court, because needs of many are far more important then needs of a few, rules that this child is to be sent to special institution for special people. Neighbours are sattisfied, parents can visit their kid whenever they want, everybody wins.
This is the USA. The only special institutions for special people they have are called prisons.
And that wouldn't even be needed if basic mental health care was provided. Screaming outside isn't all that severe for low-functioning autism, chances are fairly high that with the support of a cooperative and affordable mental health care specialist you can see great improvements.
But this is the USA. Mental health care is at a third world level.
So, most likely you've made a somewhat accurate prediction. People in the area will file an official complaint of some misbehavior of the child, committed not out of any ill will but as a simple symptom of his disorder, the courts will get involved and he'll be send to a special institution for special people, jail.