as she is so young she can’t retire here. If she has a specific skill and can get a job here then it difficult for her to be here. If she is a teacher then she will have no problem.
It will not be able to use the Bangkok bank app as it will not recognise the devise. I do this back home and luckily I kept my old phone and use WiFi to do anything I needed on the bank app
When back in Thailand just go to the bank they will sort it out on the new phone.