I'm Lucy, and I'd be happy to answer your questions today.
Unfortunately, no. A landlord is not liable for the criminal acts of a third party, even when that person is a friend of the landlord. The duty to provide a tenant with quiet enjoyment of the premises only extends to the landlord not entering unless allowed by law and maintaining the premises in a fit and habitable condition. If a landlord's minor child were causing problems, that might be different because of a parents obligation to control a minor. However, the landlord has no obligation or ability to control another adult, and is therefore not liable for a friend's illegal acts. What you can do is call the police any time this person is on the premises selling drugs. Or you could choose to vacate the premises and move, but there could be financial consequences with that unless you're able to find a subletter to take over your lease.
I will note, however, that there's a difference between the LANDLORD and the MANAGER. The landlord is the person who owns the property, and they may be interested to know if their manager is allowing known drug dealers to wander the premises and scare tenants away. If the person managing things isn't the owner, try getting the owner's name from your lease or the property records and see if you can reach out to that person.
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