I have a Usa passport with a non o visa based on marriage. My daughter has a Thai passport and a Usa passport. I would like to bring her from Thailand to visit family in the USA. Which passport would she use to exit Thailand? I am pretty sure she would have to use the USA passport to enter in the USA but I am asking because she has never had no visa to stay in Thailand because she is a dual citizen but my concern is 1. If she uses her Thai passport to exit Thailand they will not let her fly because she does not have a visa to enter the USA? or 2. If she uses her USA passport to exit Thailand she will be charged a huge fine for an overstay? Is that even possible? Thank you

She would show both passports when checking in for her flight to show her US passport to prove she doesn't need a visa to enter the US.

She would only show her Thai passport when at departure immigration. 

Then of course enter the US with her US passport.

UJ (as always) has it spot on.

 

One thing to note as a dual national. An unwritten rule is never show both passports to an immigration officer, it confuses them and can lead to a "situation".

 

For everything to do with the flight (including booking the flight), she will use her USA passport. The Thai passport will only be shown at flight check-in as the reason to why she does not have a Thai visa for her return. For Thai immigration, she will use her Thai passport only. Thai immigration does not care at all where she goes or where she comes from, only whether her passport is valid to leave or enter the country, therefore she will use the Thai passport only on both leave and entry. That is how my neighbor does with her dual passport daughter.