I've made no secret I have a family member with severe Autism. This mom is doing a good job and the daughter honestly isn't that extreme. I've seen kids kick out windows over a Happy Meal (though thankfully, not my family member. They're pretty chill and if they do have a meltdown, he isn't destructive or harmful to himself or anyone else). The girl may be nineteen, but mentally & emotionally, she isn't.
That being said, I try not to judge people who say "I could never live like this" because that's fair. It isn't a life for everyone. To a lot of people, living with a severely disabled child is terrifying. But I would only ask that you try not to judge the parents who are doing the best they can for themselves, their families and their child.
And I understand being terrified of having children with disabilities. That's why I've made the choice not to have kids. Not only would I be considered a geriatric pregnancy, but since I have a close family member with Autism, the chances of my having one increases, due to family history and age.