Depends. If he was really young I would reprimand him and let him know and give him consequences. Unless I saw that he/she was mature enough to handle it and make responsible decisions.

If he was older I would do what my dad does..let me do my own thing, even if I'm not smoking just let him/her know that there are consequences and if these consequences ever come to play that he/she is fully responsible since they feel they can make the decision to smoke weed.

I wouldn't smoke with him/her until he/she was over the age of 18 just because I don't want CPS or DCFS on my ass.