I'm not a lawyer, but it's already been said, /r/legaladvice is a good resource.

Anyways, there is very clear-cut routes to get around this. First off, read or search for key terms in the Manitoba (Residential Tenancies Act)[https://web2.gov.mb.ca/laws/statutes/ccsm/r119e.php]

Without seeing your exact contract, it's difficult for me to help you beyond the contexts of this act. One thing I noticed right away is:

Obligation to take care and repair damage 72(1) A tenant (a) shall take reasonable care, and ensure that any person he or she permits in the residential complex takes reasonable care, not to damage, wilfully, negligently or by omission, the rental unit or residential complex, including anything relating to a service and facility or a tenant service provided by the landlord; and (b) subject to subsection (2), shall repair any damage in a good and workmanlike manner, or pay compensation to the landlord, within a reasonable time after receiving a written notice to do so by the landlord.

Considering you also said this in another comment here:

I do not have the legal capacity to do so. He is a cosigner of the lease.

Add the two together. You did not permit your room mate in to the property, that was the land lords decision - not yours. He is not your guest. You have no control over his presence.

You need to also consider that many courts have already ruled that Canada's laws restricting cannabis have no effect. It's very grey legal territory, not something a judge ruling on a tenant is going to overthink.

Given this fact, you seem to have no liability. If you were put in front of a panel or a judge, and you raised the fact that "ratting out your roommate, who you never wanted" would only make your life stressful and perhaps your own home unlivable by creating conflict, I can't see anybody reasonably punishing you for your decision of silence.

It also seems like you have no direct liability. But again, I'm no lawyer.