Start slipping Geodon in her food. After a couple weeks when it starts to work, she'll realize how insane she was acting. Only problem is there's a catch-22. . .I think you have to be insane and have a prescription to get Geodon and slipping it into unsuspecting people's food is probably also highly illegal.
Somebody I know and who I am close to started acting similar to what the OP described and eventually had to be taken (by me) to the mental ward. Luckily the person was so out of it they didn't really have any idea where we were going or the ramifications of what could happen once we got there despite my repeatedly asking if they knew where we were going and what it meant once we got there. (I say "luckily" because I didn't have to force the person to go, they were so clueless they just came along peacefully.) The person kept saying yes they knew where we were going but then a minute later would ask me where we were going. . .Anyway, they had paranoid delusions and hallucinations and the works. Ended up spending about a month in the psych ward. Sounds like what your GF's mom is having is definitely a psychosis of some sort. It can be caused by any number of chemical imbalances in the brain. If it has been going on for quite some time as you mentioned, she probably definitely needs to be on some kind of anti-psychotic meds. Seeing this person in that state was one of the scariest things I have witnessed in my life, even though the person showed no dangerous homicidal or suicidal tendencies. . .it was like watching somebody on an extremely bad trip that never ended.
It may be that the only thing you can do, OP, is to literally get some licensed qualified professional to come to the house to observe your GF's mom and make the determination on whether or not the guys in the white coats need to come and cart her away. There are cases where a judge can have somebody involuntarily committed but usually they have to display that they are somehow a danger to themselves or others. If your GF is a minor and dependent on her mom, you may have a case for involuntary committal. The problem with people who are "crazy" is that they have NO IDEA they are crazy. In fact, they think everybody else is crazy. The delusions they believe in are 100% real to them. They are quite literally living in an alternate reality from you and I. You cannot talk reason to them. Don't even try. They will talk you in circles for hours. Just get a licensed and qualified professional in the house to have a look at her in any way you can. If she's a sick as you describe, it will become very evident to them within a matter of minutes. Then talk to the professional about what your options are. Unfortunately you will never be able to talk rationally with her about her illness until she becomes stabilized. She will refuse to acknowledge anything is wrong with her probably because she doesn't even realize there is anything wrong. As far as she's concerned the only thing wrong is that somebody keeps stealing her stuff. After she is stabilized, it's very likely that she will have clear recollection of all her insane behavior and realize how ill she was. Then and only then will you be able to have a rational conversation with her about sustained treatment.