Is he willing to go? If so, of course do so. Maybe most of what I wrote can be a plan B, but probably won't be necessary.

Weed can (indeed, of course) lead to psychosis. More severe (that is, starting more severe) than the worst scenario I described. I may be especially vulnerable but there are lots of people with only a mild vulnerability that might never become BP or anything like it that may get a (brief) psychosis.

THC concentrations vary and higher concentrations don't necessarily mean a greater chance of psychosis, in my limited but for me, impressive, experience. It's weird how it works. I have no problem with hashish.