I've a buddy who has a doctorate in aerospace engineering and a minor in applied mathematics...smokes pot ALL the time. Cool guy, to say the least.

To answer your question, it is a matter of comfort and mutual respect. If you are okay with him doing it, then he needs to be equally okay with and respect whatever decision you make regarding you. A quick way to create one heck of a point of friction is for him to try to badger you into trying it if you don't want to. Outside of that, don't be afraid to communicate with him about it...are you okay with him doing it while you're around? Things like that should, I think, be addressed.

As far as generalities go, I don't smoke myself, but I've quite a few friends that do. I only ask I not be around when they smoke, and for the love of all that's holy, don't put me in a car that has it in there (NC considers all passengers in a car to be guilty of possession if marijuana is found, and my major is worthless without law school). I agree with the sentiment that I think alcohol is, on the whole, probably much more destructive than pot, though stories like Bub's do happen (which if you'll allow me to say Bub, I'm so sorry he would ever do something like that to you. ).