As one who inherited a dental condition that caused my teeth to have very little enamel, I can truly sympathize. I would say give Iolite's advice a try and see where it leads.
Another thing you could try --- if there is a dental school near where you live, your GF can go there for a consultation. Dental schools tend to charge 1/3 of what dentists charge. However, do note that they usually only see patients M-F 9 a.m. to 5 p.m. Plus it isn't a matter of just taking a whole day off to get the work done. She would have to go for several different appointments. I know about this. Years ago, I had a co-worker who went to a dental school to have her work done. I made an appointment with the student who was working on her. He told me to expect to take lots of time off work. In addition, he told me that the work I needed didn't fit into his curriculum! So, I didn't go back. However, many people do use dental schools with much success.
If doing that isn't feasible, I would get a second (and even a third) opinion regarding the work needed. What I have found is that some dentists see dollar signs in their eyes and only recommend the most expensive procedures.
Your GF knows her teeth best. In the past, has she tried to save her teeth only to have them crumble anyway? If so, sad to say, expensive work may not do the trick. One of my brothers has the same inherited condition that I have and over the years he's spent thousands of dollars trying to save his teeth and it was all for nothing.
After your GF has gotten more than one opinion, she can evaluate her options. If your dental insurance is decent, then it should cover some of the work, right? If not, I wouldn't call it "decent" if it won't at least cover some of it.
As for payment---she could pay off the work over time. These days, dentists will arrange for loans---even charging interest! However, if she doesn't like the terms the dentist is offering, then perhaps her bank can offer a better rate.
Finally, I hope she doesn't think that everyone who sees her teeth judges her poorly. I, for one, wouldn't. I also understand that many people, in addition to heredity, have poor teeth because they can't afford to fix them.