Yeah TBH, your friend could have developed OCD with or without cannabis consumtion.
If you take any person who has developed any sort of mental problem in their life, and look back over their previous history or drug/alcohol use, chances are 60-80% of those people will of at least tried some form of drug, but this does not nessecerily mean that this drug is the cause.
There have been studies that I have read that show that when cannabis use increases within a population, mental health issues do not.
I cant remember where I read the study but even with increased cannabis use within a population, the number of people who developed shizophrenia and other mental health issues stayed the same.
If, for instance, you check on the figures for the amount cannabis use has increased in the UK in the past 50 years, and compare that to diagnosed mental health issues, you would expect that as one increases, so does the other, this is simply not the case.
Im not saying that cannabis cant bring out underlying mental health problems, but no more so than drinking alcohol can, thus I see it as a somewhat double standard that one is legal, while the other is not.