Nicola, what John Catapano says is the key for a schengen visa. Prove enought she goes back.
Also a bit of proof is that you know eachother quite some time. (photo's).
And she needs a passport ofcourse.
I first went to my municipal in the Netherlands let them stamp a form that my income was high enough to support her and give her accommodation in my house
(My Employer's declaration). Maybe smart the first time, only get her 3 or 4 weeks. Not the full 90 days.
Then I made an invitation letter with reason I wanted to show her my country and introduce her to my family. Also reasons that she has to go back. Also my phone number and that they could reach me
****
.
I also gave 6-7 photo's of me, her and her family. She also did not have a real (tax) job, but helped in a shop and helped family on the (rice/casave) land. Make foto's ! Not so many Thai people have 'a real' job that they have to pay tax or have healtcare insurance by them.
Only if they work at a bigger company.
Then does she have younger children or older parents she has to take care of (so she has to go back), does she have a blue house book, owns land etc. Find as much reasons as possible that she has to go back.
Book a refundable ticket and at last a Thai healthcare/travel insurance for the time she is with you. Then there were 12 or so recomended companies.
Before one had to go to the Embassy for some kind of interview. Now it's done by VFS Global I think. Both Bangkok. I got even a tip that she dresses properly if invited for the interview.
Sometimes it seems to be a bit nitpicking, but in this case the more proof the better.
Good luck.