How about a serious answer?
You have two choices Evangelist: Deal and tolerate or seperate. that's it.
My mother was your girl's mother. mine was mentally ill as well but that doesn't change what I'm about to tell you. My mother claimed my now sister in law of 30 years! was drugging my brother. She claimed my now husband of 24 years was selling drugs. And that was just the tip of the iceberg and we didn't have the unwise decision of sleeping in the same house without a chaperone to deal with.
Some people are simply SET that their way is the ONLY way and that you either aren't a Christian or God is going to get you good, if you decide to do things differently than they did. We dealt with that kind of stuff all.the.time. My parents wanted to tell us: where to live, how to spend our money, forbid us to do things such as go see movies or play cards (even after we married), you name it, they knew how to do it. And they knew how to lay on the god-guilt if we dared stray from the path.
We didn't want to die and go to hell did we? They were only concerned for our souls..... yeah right. They were concerned with keeping control because they didn't know how to live without it. They had children to serve and entertain them and they wanted to keep things that way. And of course God expected us to honor our parents didn't He??? Oh yeah, they were good.
Now, here's the thing. YOU CAN'T CHANGE HER. Your girlfriend can't change her mother! Mother has been this way for YEARS, her actions and personality are set in hard and fast. Hence the choice: seperate or learn to ignore her. I've done both over the years.
Seperating meant boatloads of guilt that had to be worked through even though I KNEW my parents were wrong. It also meant I had HUGE trust issues becasue if you can't trust your parents who can you trust?? Are you going to be willing to stand by this girl while she goes through what I promise will be intense turmoil?? If not, break it off now.
Dealing. Dealing was just as hard. It meant *I* had to set boundries with my parents and stick to them. And sometimes sticking to them meant seperating from them for long periods of time (reread the last paragraph). You can't set those boundries for your girlfriend. You can encourage her to maintain them, but SHE has to do the work. And its hard. Especially when they call up and say how much they miss their baby (which is an insult in and of itself because it implies she's still a child and incapable of running her own life, I was probably 40 the last time my parents tried that line on me).
It's tough and then they get old and NEED someone to help them. This sort of thing is the never-ending story. You have to develop a thick skin and say "I'm not going to reply to such drama. I'm not going to escalate the power trip. I'm not going to allow you to hurt my feelings." And then you have to be willing to love your wife(if you marrry) when she can't say those things and breaks down in tears yet again. And if you can't, you have no business marrying her.
