[Neilk]

Originally Posted by David

...

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.

That's what I do with my French passport when going to Europe. I guess I can leave the French passport at home when I go to England this summer since the UK booted themselves out of the EU.