Every time your mother wants to say something negative about your father say "La Hawla wa La Quwatta Illa Lillah". Remind her that everything in this life is a test and all power in the Universe belongs to Allah. If she is upset about it, she should take her matter to Allah and ask Him to guide her. Pray to Him that He fixes the issue between her and your father.
Maybe you should suggest to both of them that they need to fix their problems for the sake of Allah and perhaps they should get a mediator or go to an Islamic counselor that than help them get through their problems. If they just bad mouth each other and always stay angry, they will never be happy.
Also, don't let your mother say anything bad in front of you. Remind her Allah is watching and that no one is perfect. Constant remembrance in Allah will help, Inshallah.