If it does not effect you then mind it. Who the hell are you to tell someone what to do with there lives. If its coming from a place of love then great. take it to pm's. If its coming from a nosy place, from a I'm better then you place then shut the hell up.
Honestly there are worse things a person can do then smoke pot. Omg I would be more worried about heroin or coke or meth or MDMA or say alcohol. In the end its there path to choose not yours.