- Denmark for a long time didn't allow dual citizenship.
Its highly likely she would have had to relinquish her Thai citizenship before she got her Danish passport. She would have renounced it under section 13 of the (Thai) nationality act, and that renunciation would have been published in the royal gazette.
http://web.krisdika.go.th/data/document/ext810/810050_0001.pdf
- It is very possible for her to resume her Thai nationality under section 23 of the same act. She will need to apply for this via the police special branch in Bangkok who should be able to give her the correct information on how to do it. There is a process, it will take time, but its entirely within her rights to do so.
- In the meantime, under immigration order 327/2557, section 2.23 states that "In the case of a person who used to have Thai nationality or whose parent is or was of Thai nationality visiting relatives or returning to his or her original homeland" they can apply for a special non-immigrant visa which will effectively allow her to stay here on her Danish passport while her Thai citizenship is reinstated.
There is a good article about that here: www.thaicitizenship.com/thai-ancestry-visas
- until her thai citizenship is reinstated however, it is likely she will remain outside the Thai social security safety net.