You would have to have it notarized in the US Consulate. In Thailand there is also something similar ("pipat") to handle matters in Thailand, but maybe a wife wouldn't need that. I'm not sure, but a court intervention might be needed. Logically the durable power of attorney would be created under the law of the state where the person last resided before moving abroad.
Here is an explanation of DPA under Florida law, for example:
https://afela.org/elder-law-faq/florida-legal-information-by-the-academy-of-florida-elder-law-attorneys/durable-power-attorney/
The acceptance by the person(s) acting on behalf could be done before a notary in the US in whatever state he/she/they are located.