Before throwing stones, you may want to introspect on the fact that the parent never said “homeless” but rather “mentally ill” and you just assumed that’s what he meant
What I was actually alluding to is https://en.wikipedia.org/wiki/O%27Connor_v._Donaldson and https://en.wikipedia.org/wiki/Addington_v._Texas
From my non-scientific, but rather large sample size of these mass transit experiences, I would ballpark unmedicated schizophrenia rates in excess of 50-60%.