Agatha Christie's Ordeal by Innocence

Back from a long trip, Dr. Arthur Calgary (Donald Sutherland) seeks to return an item left in his car by a young hitchhiker - Jack Argyle. Except Jack is now dead, executed for murdering his mother...a crime he didn't commit, as he was in Arthur's car at the time of the incident. Arthur could easily clear Jack's name, so why is there no one in the young man's family that wishes to reopen the case?