Having people in my life who are bi-polar, I can only speak to that condition, but often people with these conditions, though troubled, are still very lovable. They can often be exceptionally intelligent. And caring.
But then the disease sets in. And having seen people I love flip out, when I wasn't prepared for it, you simply do not know what to do. Most of us are not trained to deal with serious mental health issues.
And schizophrenia is often not apparent in the early years of someone's life. It can appear to pop up out of nowhere. The ATV injury may have taken his illness to a whole new and unexpected place. His parents know him as an infant, a toddler, he is of their flesh. They may have believed there were things they could still do to help him have a healthy normal life--he is very young.
I doubt that in their wildest dreams they ever saw this coming.