I think you have no clue. I am willing to bet you have alot of 'friends' who are pot smokers but do not disclose that to you because of your views towards it. I am willing to bet at least a few of the professors there are pot smokers, but since it is illegal to use they will not ever disclose they smoke weed.
Back in the 60's when weed was out there everywhere the government tried to pass weed off as a death drug that makes you go blind, which has since been proven false. I have never heard anyone 'smoking' themselves to death like other drugs can do these days.
I can tell when someone has never tried weed before. They are under the impression it is a life ending drug that will make someone homeless and jobless, which isn't true. I know several 'high profile' people in my community that smokes weed and has a huge job. Just like beer, you need to know when and when not to use.
As long as he is respectful enough to no bring it into the dorm, DON'T WORRY ABOUT IT. IT ISN'T YOUR PROBLEM OR ANY OF YOUR BUSINESS WHAT HE DOES IN HIS OWN FREE TIME, EVEN IF IT IS AGAINST THE LAW.
BTW, I am an ex'pot smoker and can tell you first hand, it does not make you go kill people or rob people for it. It is nothing like any other drug in the sense that it makes you go mad if you do not have it.
If your friend wants to spend his money on a bag instead of a movie, then let him!! It is his life and you do not have any right in the world to interfer in another human beings life unless it is directly affecting you, which it sounds like it isn't other then the knowledge knowing he does
Just look at it this way, If it was soooo bad, then cops who pull people over and find just a little bit and make the owner of it dump it out and rub it into the ground would instead be sending them to jail and throw the key away, which for the most part they don't unless you are a prick to the cop.
Also, The Brits just passed a law that makes pot posession a ticketable offense instead of an arrestable offense. The US usually follows suit with Britian in it's laws etc... so I am actually expecting pot to be 'not as illegal' in 5-10 years.
(Steps down from the soapbox)