by having his gun permit around housemates who smoke weed.

Federal law on form 4473 states anyone who smokes is ineligible for possession of a gun despite it being legal in some states now.

What is legal under state law is not always legal under federal law.

Would the issue of constructive possession be in play because he knew the roommate smoked weed and there would probably be some in the apartment, making him some kind of co-possessor?

The state statute you quote, lists six specific exemptions from state prosecution. Your "friend" should ensure that at least one of those exemptions is applicable at all times.

albeit locked up in the apartment around a guy who is a marijuana smoker, making him a possessor of a firearm by a prohibited person?

So, you are trying to argue that (in the eyes of the court, or prosecuting attorney), that a firearm locked up in a container, or disabled as per the exemptions in accordance with state law, is equal to "possession"?

Anyone have any advice/rebuttal on this issue?

Talk to a lawyer licensed to practice in your state. Make the phone call for a free consultation.