Sure, marijuana may exaggerate seemingly innate personality characteristics, but create them? Highly unlikely. Its all about the individual's personality and mindset - same reason some individuals develop PTSD, while others walk away from the exact same traumatic experience with no lasting psychological scars.

I feel confidant in saying that there are far more chronic users of marijuana who are otherwise mentally healthy than there are users who are teetering on a "major psychotic episode" - a sign that people such as your colleague are the minority, unstable demographic of the population. Also, it is far easier for individuals who 'freak out' and have embarrassing 'psychotic snaps' to point to an inanimate, socially demonized substance to blame rather than to find and accept blame with 'self'.

"... many people believe that marijuana causes mental illness, in particular, schizophrenia. Several correlational studies have shown that heavy marijuana users are more likely to be subsequently be [sic] diagnosed with schizophrenia... However, there is no way for correlational studies to rule out the possibility that the correlation between heavy marijuana use and schizophrenia occurs because people who are predisposed to develop schizophrenia also tend to use marijuana."