I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to her that this happened.
No, a landlord cannot just lock a tenant out, even if they owe rent, and even if one of the tenants get arrested. The landlord must serve you with an eviction notice, wait to see if you pay the rent (or leave on your own) and if you do not, then the needs to go to court. You do not have to leave until you have been served with notice, and if he tries to change the locks, you may be able to get a police officer to let you back in. The police actually have authority to arrest a landlord who blocks a tenant from accessing leased premises - even when the tenant is behind on rent.
On top of that, you'd be able to sue for any expenses incurred during an illegal lockout. If for some reason police are not able to help you get back in, call your local tenant's rights organization. They should be able to help you seek a court order requiring that the landlord let you back in until he can evict you properly.
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