Financial talk

Can my mom and her boy friend live together and still not have the state view it as a common law marriage.? This isn't about $$$ but about lack of trust. My side doesn't trust his family in any matter. Money, relationships, parenting skills. Anything. Can they just live as landlord and renter? House bought in '92 200K stilled owed 100K. Today's value 500K. Age in the 70's and 80's