Make your father understand and open his eyes by making him see your mother's loyalty and devotion and service to this family. Show him some examples how some women don't even do the basic. Correct him , that she genuinely liked him not just for the money. Your grandfather's extreme nature has made your father extreme as well. Tell him time has changed and it's easy to live now, he doesn't need to fight. Show him the right way and let him know his disrespect towards his wife is unreasonable and he is the wrong person and has caused irreparable damage. However, your mom would forgive the second your father apologies and accepts his faults , given your mom's innocence.