[David]

I'll add another UK funny aside: I have a friend who grew up in East Texas, has the East Texas drawl when she speaks and for all intents and purposes is a Texan, but she was born in the UK and maintains a UK passport.

Last time I went to Thailand, pre-Covid, I went thru the Thai citizen line with my wife which went pretty quick. Not sure how that works today.

So it sounds like she should:
Buy the ticket with her US passport.
Leave the US with her US passport.
Enter Thailand with her Thai passport including any paperwork she fills out before landing.
Check into the flight in Thailand back to the US with her US passport
Enter the US with the Thai passport. (Meant US passport )