Have a formal lease between you and your mother. Make the lease on a month to month basis. Make the lease for fair market value but do not collect from your mother. If your mother dies, you give 30 days notice to the man to vacate. If he does not, evict him.
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I agree with Mr. Fink get something in writing that makes it clear the boyfriend ha no rights as to tenancy and is simply a guest who can be asked to leave at will. However, your Mom may not like the idea.