At the visa interview they will evaluate if she has enough ties to Thailand to keep her from overstaying in america. 99% of the time it will be denied, and they will tell her to have her us citizen boyfriend apply for a K1 marriage visa
What ever you want to provide as proof, your home country embassy has affidavits they can sign for you. But for freelance work what would you show id you were a writer, painter, web designer, etc