If there really is a judgment, you might be able to have it revisited since you did not receive proper notification and thus were unable to file a defense.
While I am somewhat familiar with the Tenant Protection Act in Ontario, I have no familiarity with the landlord-tenant laws in QC, so I don't know how it would work with respect to the roommate issue. I would guess that everyone on the lease is principally and severally liable - so yes, unfortunately you'd be left holding the bag (the whole bag) if they have all filed bankruptcy.
He should not be holding back information. You should send him a letter asking him to provide the strictest proof of the debt and the judgment.