My parents have a multi-family house. There is one woman who has been a tenant for at least a decade.

there is no lease agreement.

she is not working, but she is collecting disability. she has a young child (8ish) and he may have a learning disorder. Her ex-husband sends child support.

there have been many month where she's been late on payments or only paying a portion of the payments. my father has only raised rent a few hundred dollars over the years because he feels bad for her. she is currently up-to-date on payments.

my father recently tried to switch insurance companies and when the inspector came to check her unit, he claimed he couldn't do the job because her apartment was FILTHY and filled with trash. the woman has piles and piles of old newspapers and magazines. her son's room and bed were also filled with garbage.

my father has since spent his own time going to her place and filling up bags of garbage to throw out. the woman keeps protesting saying she'll "take care of it herself", but she refuses to do anything.

how difficult would it be to have her evicted and on what grounds? is it as simple as terminating her tenancy-at-will? her apartment doesn't seem like a sanitary place for a child and a small dog that she recently "got as a gift" during the time when she couldn't pay for rent.

thank you in advance for any advice.