Yet that is one of the duties of a doctoral advisor (yes, I know we were talking about a master’s thesis, but it’s the same idea). The doctoral advisor offers advice and opinions about the content of the dissertation.
I was hired by a doctoral advisor to edit someone’s doctoral dissertation. The dissertator had done good research, but had difficulty with the chapter on the literature. She gave me the literature (stacks of photocopies of journal articles), and asked me if I could read it all and finish the chapter for her. I did. She paid me out of pocket for that additional work. The chapter occupies about a third of the main text of her finished dissertation.
I am credited in the dissertation as her editor. I don’t know whether her dissertation committee knew the extent that I worked on the literature chapter, though. Not long ago she wrote a very nice letter of recommendation for me. But it made no mention of my working with her on the dissertation.