actually, the owner of an illegal apartment must tread lightly…he has much more at risk than the tenant. The landlord can suffer heavy fines from the government, risk not being able to collect rent, be ordered to go through the zoning appeals process, a a great cost, to make the apartment legal, and/or be ordered to make major repairs/renovations to the building.
Giving legal advice without being a member of the bar is illegal in most states. You haven’t any knowledge of the lease or the tenancy laws in the jurisdiction. In many leases, and in many jurisdictions, a tenant is required to provide the landlord with keys that work to open the apartment for security/safety purposes. The tenant may be required to receive permission or be forbidden by lease to change locks.
That $20 lock could cause thousands in legal expenses for the tenant if sued for eviction because of breach of contract, or costs of new locks, door, frame, etc. if the door must be broken down for emergency access because the tenant changed the lock and didn’t give the landlord the new key.