What passport should my Thai-American wife use when buying airline tickets and traveling to Thailand?

It doesn't matter what is in the ticket, if you have a passport that matches the info. And despite what others say above... There is no connection between the ticket/passport you use and the passport you show at immigration.

She should travel as a Thai citizen as does my wife.

I never had an issue entering my wife’s Thai passport number and Thai nationality for trips to Thailand including our last trip this month.

Make certain the passport used to enter Thailand matches the passport number on the COE.

The US passport all the way but show the Thai passport to Thai immigration when you arrive.

I think your question is not about what passport to use when entering Thailand as others are answering, but rather what passport to use for the airline ticket and at check-in. This is covered a lot in the various travel forums regarding dual-nationals traveling around the world. The US does not have exit immigration clearance, so you should buy the ticket and check-in using her Thai passport. Otherwise, using her US passport can create difficulty at check-in because if the airline enters her US passport, it will alert them that they must follow all the entry requirements for US nationals to enter Thailand via visa exempt, and she may not meet all the requirements (return ticket, insurance, etc.). Technically, if she used her US passport name, they can override with a notation that she has a Thai passport as well but the problem in this covid era is that airlines are super strict about their personnel following exactly the entry documentation requirements listed for whatever passport is shown in the system, which in this case is US, so it will likely cause significant delays involving calling supervisors and arguing with them, so why do it? She can continue to show her Thai passport to get through TSA and at the gate, since as I said, the US does not have exit immigration so all they care is that the passport name matches the name on the ticket. In the extremely rare case that CBP officers do a spot-check at the gate for illegals, she just needs to show the officer both passports and they will notate it in their system.

You should use her Thai passport information to buy her ticket. She will leave the US on her US passport. Enter Thailand on her Thai passport. She should travel to Thailand using her Thai passport.

She must enter Thailand on her Thai passport. If she enters on her US passport she will be subject to visa and 90 day reporting requirements as a foreigner.

We entered last week. Thai spouse used her Thai passport and I used my US passport with a visa. COEs are required for us both, but otherwise, no issues on arrival in Phuket.

why not ask the proper authorities ie: airport,immigration , embassy etc

My wife comes to Australia on her British passport and leaves on her British but must enter re enter Thailand on her thai passport.

She can use her Thai passport to enter Thailand and leave Thailand...

And use her American passport to leave the USA and reenter the USA.

But regardless of what she does... She will need to obtain a CoE certificate of entry from the Thai Embassy in the USA to fly to and enter Thailand.

If you are both fully vaccinated you will have choices to make in entering Thailand.

Wait for the announcement of the new entry requirements for Thailand in coming weeks.

Well i presume she used her US passport to enter the States so she should use her US passport to Exit