What about just telling her, "Mom, you can't change him so you'll need to find a way to let these things go. If X is important to you, then great. He's his own person with a different set of values and opinions. It's ok to share the same space and not share the same ideas." Then ask her, "So what did you do to make yourself feel better?" Complaining to dad is not a solution. It's controlling. Venting with you is not a solution. It's crossing boundaries.