@qbteachmt " his right to the property should be protected within the life estate, and she cannot make him lose that,"

She's the one with the life estate.  He has what is called a "contingent remainder."  You can look it up.

Maybe this is a tax question.  But my guess is that it's a squabble between two people who would never have met had it not been for a third person, now dead.  Is the son really interested in a tax deduction for rent?  Then let him claim it.  Is he really interested in complying with 1099 rules, but doesn't want to upset the owner of the life estate?  Then tell him he might have to pay a penalty in the unlikely event of an audit, but the deductibility does not depend on the 1099 filing.  Or does he just want to stir up trouble with Dad's significant other?  Then he doesn't need you, or a lawyer.  He needs a psychologist.