I don't see the problem, except it being illegal in some states.

I don't smoke pot, but can tell you for a fact I know tons of professional who use it. Most of them are productive people with successful lives and never felt the need to try any other drug. In fact, many of them rather not drink.

Pot (they say) relaxes them and helps them be creative. In fact, I have a friend who had a very bad tooth ache, and a marijuana tea resolved the pain. Its medical attributes are well known and are allowed for medical reasons in some areas. For me, it is absurd that it is illegal, honestly.

My advice to you: watch how he uses it and how much of an impact it has in his life. If it's too much and likes to be high all the time, it might be a problem that you're not willing to deal with. If he consumes other stronger stuff, watch out.

Also, watch the laws regarding pot in your area and ask him to be in compliance in order to avoid any situations that might end up embarrassing or damaging you. Its only fair!