- you clearly have no idea who gets visas here and who doesn't. Wealth or independent means are what matters. People who can demonstrate they have more reason to return to Thailand than to remain in the U.S.
- long wait though. Could be 2 years. Examination of the veracity of the length and seriousness of the relationship is part of the vetting process and in separate interviews. If they are definitely serious, yes, they should try that. No guarantees with that either.
Good luck. As high as 90% rejection rate for young love applicants. She would have to show overwhelming evidence that she has more reason to return to Thailand than to remain in the U.S. Land, property, investments, standing in the community, education - and (here's the catch) a good job with salary and benefits that she has permission from to take vacation leave to travel. Everything presented will be checked and double-checked, so no need to try and stuff her bank with a large deposit. This is a very long shot, so look at what Plan B will be.
If we allowed visa exempt status for Thai visitors to the United States, there would be no one left here except for us expats and the hi-so Thais and their millionaires. No one left to serve us in restaurants, no nurses in the hospitals, and no party girls in the bars. Service in the U.S. would be much better! 555