HELPPP PLEASE )):?

My uncle owns a 2 family house and he installed a kitchen into the kitchen room in the attic. I didn't know that was wrong since it's your house. But anyways, a family of 4 are renting the attic for 2 yrs & lately they haven't been paying the rent according to my uncle. He is so worried & aggravated because he has so many bills to pay. So of course, he went to tell them that & the attic people decide to be idiots and threaten him to tell the cops & city hall about it. They basically want to live in there for free. I cannot believe this ridiculousness & stupidity! They even got a lawyer friend so I'm worried that the law might be on their side )): My uncle has enough to worry about but I wish I could help. What can he do to get them out asap?