Catch 22?

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If it's not adversely affecting his behavior then I personally would keep quiet. If he becomes irresponsible again (that is, neglecting his obligations) then you'd have justification to express belief he was toking without admitting you were spying, and the entire point of getting him to quit was to curb his bad behavior, not the smoking itself. Hopefully, his habit is curbed enough now that it won't negatively affect him anymore.