The answer to your last question is yes. Thai name for thai passport. No reason to change her Thai name on her Thai passport.
The question you initially put forth was changing her Thai name to an Eng-lush "married name" for a UK passport. Obviously that is where the problem lies....changing her name (identity) to obtain a passport in the UK. Obviously she can apply for one with her Thai name intact with very little hassle...(or has that water already passed under the bridge ?)