The car has (had) to have been owned/registered in her name for at least 18 months before she moves back to Thailand.
If the car is imported in her name, under a declaration that she is moving back to Thailand to live, and it then turns out that is false and that a foreigner is the full time possessor/driver of the car after she returned to her family overseas that would be an illegal import and the risk would be that the car would be confiscated and you may be prosecuted.
Having said that, I'm sure that legislation was introduced a few years ago to ban the personal import of all used vehicles.