What can I say to help my wife get out of the dumps?

My wife and her Mother are not on speaking terms right now. I don't think that's going to change any time soon. Her Mom is certifiable, and never owns up to the stupid crap she says and does. For years now, my wife has tried to get her Mom to understand that the way she acts is disrespectful and embarring, and that her lying all the time pushes people away. Because of recent events, they had it out this week and stopped speaking. Even though my wife is furious with her for the way she's been acting, she also feels sad about the whole thing. Her Mom has been like this her entire life. One day she's nice and motherly and the next day she is your worst nightmare. She hasn't been there for her, and very rarely sees our kids by her own choice. I know for a fact that my wife has always tried to resolve their problems, but her Mom won't meet her in the middle. They are both grown and I get that. I'm not getting in the middle of their problems. I only wish I knew what to do or say so my wife would know that she can't make people change and that she shouldn't feel bad about an idiot who could care less about her. Right now especially I know she needs her Mom, since she's pregnant with our third child. She says she's fine, but I know she's sad that she never really has had a dependable, loving mother. Now her sister is taking sides with her Mom since their Mom pays for everything she has. Any ideas on making her feel better?