She might try telling the embassy that her purpose for coming to the US is tourism. To see DC or NYC. You should not be seen by anyone at the embassy nor should she mention her American boyfriend.
Based upon my experience, this is quite true. My youngest stepson (in his early 30s at the time) was denied several times. No spouse, children, job, money, or property. US Embassy knew it was a ploy by his Mom to get him with us. Eventually, he got his green card.
You are right. In process of getting a replacement diploma. I have had meetings with two schools in Chonburi that were set up by my stepson. Getting the diploma is my first priority now that I am in the US, or somehow locating the original. Thank you Kool Breeze!
Back to the home country to cool off and regroup for a few months. Then a METV issued by the Royal Thai Embassy DC or Consulate General could work her, as there could be a significant gap between her previous entries and the new visa.