However, I think you missed the main point of my post, which was to suggest that perhaps there was a comorbid reason for his perceived cognitive deficiencies that were not a direct result of marijuana, but related to known side effects, such as REM sleep disruption.
It sounds like you feel personally threatened and you feel some need to shoot down the well established scientific fact that drugs impair cognitive function.
There may be more going on in his life than marijuana impairing cognitive function, but you have absolutely no basis whatsoever to assert that they are wrong about their own life and there is some other explanation for the cognitive effects they experienced.
I don't smoke much anymore, but more because I don't feel like I have a safe place to kick back and smoke than anything else. When I do smoke I feel tired the next day.