You write that "he had a few problems that hadn't manifested themselves yet". It sounds like he would have eventually reached the same behavior pattern even had he not had that experience with mushrooms.
He may not have. Most psych issues have a trigger i.e an emotional or stress experience. Something that violently changes your brain chemistry has the same effect.
As I understand it, things like schizophrenia are primarily hereditary, and the trauma pushes you over the edge. So, people with a family history of schizophrenia or related conditions shouldn't do these drugs.
Still, sad to hear.