Your mother (or her estate) and X were sued and lost. X was a beneficiary of her estate. The judgement has now been paid in full as mother's house was sold to pay it off.
Now X wants his share of the estate as if he had no part in the suit or debt that Mom owed.
It can vary amoing states but if X was the person primarily liable, and Mom had a claim against him for indemnification, then his share would be offset by what he owes Mom or her estate in most jurisdictions, although the process varies.