I think a lot of good points have been made. Obviously pot isn't the worst drug in the world, but it is currently illegal. From a personal perspective, I could care less if the legislature legalized it or not. I've never even tried pot, and never had any desire to (I'm kind of a health nut). But, regardless of anyone's opinion, it is still illegal now.
From my LEO perspective, I can say that I don't think you'd be very likely to run into trouble if he was busted with an ounce or less of weed (a city ticket where I work, which is charged on the state violation).
But, if he is selling, growing, keeping large amounts, etc... you may find yourself in a heap of trouble that you don't want.
Overall, it is a bad arrangement because of that risk, and I'd probably stay clear of it. FWIW, in my area of the country pot isn't taken very seriously these days... It is still illegal, but many times people don't even get arrested (or a summons) for a small amount of weed.
Other parts of the country take it far more seriously.
All of that aside, I wouldn't even consider this arrangement if your friend was into anything more serious!
Sometimes you need to make choices for your own good, and sometimes those choices aren't ideal. I've always been active in mountaineering/climbing (sort of a hippie culture), and as such I've had friends in the past who smoked weed... With my LE career these days, I stay clear of those old friends. The ones I am still in touch with know that they need to do that stuff away from me, and not have it around me (I am not willing to risk my job due to their actions, and the ones I still consider friends respect that).
By the same token, you are a lawful gun owner... Don't put yourself in a bad situation just to save a few bucks on rent!