It would help a whole lot to know if he is just charged, or if he has been convicted. If he has been convicted, it's rather late to be asking what the laws are.
I'm in the US. It wouldn't make sense for drug defendants to be able to just say "those aren't mine" and they get off. In the US, and likely in Canada, there is something called "constructive possession," differing from actual possession. If the cops find the MJ loaded in the pipe that is in your mouth, that is actual possession. If the cops find the drugs under your seat in a car, that is constructive. Basically, if they need additional evidence, like a confession, they are proving constructive possession.
In reality, the distinction doesn't matter because the ultimate question is the same - do we have enough evidence to prove the drugs were in the possession of the defendant.
In your friend's situation, unless someone is coming to court to admit the drugs are theirs, your friend would often take the hit on the charges. A lot can depend on where the drugs are found, however. If they are in the console on top of your friend's wallet, that's evidence they are his. If they are in the back seat in a suitcase, in a bag with your friend's passenger's unopened mail, that's evidence they might not be his and might belong to the passenger.
I cannot count how many times a client has said "Friends left those drugs in my car." Those clients were usually not willing to go do a drug test to help me show they do not use drugs.