You should find out exactly what is behind this... if she has engaged in prostitution or even worked as a bar girl, it's likely that her petition would be denied. Engaging in prostitution is one in the eligibility list.
If she has been married to this guy and the divorce is final, she is fine. If she was in a relationship, no wedlock, he has no right to claim anything. Thailand does not recognize domestic partnership.
The US consulate doesn't take any allegation without concrete evidence, so
if she's never engaged in sex trade, nothing to worry about.
You may have her contact me directly at the forum on my blog at
Lawanwadee
I also speak Thai..