From what I understand when a woman goes to the mikvah there is another woman who watches her immerse (dubbed 'mikveh lady'). Apparently this is for practical reasons to make sure everything is done properly (Igros Moshe Yoreh Deah 1:105). I would like to know if there are practical halachic ramifications of this practice, such as does the woman immersing have to listen to the mikveh lady if she is told that she made a mistake?
For example Sarah watches Esther go to the mikveh. Esther comes out and Sarah says, "Esther you touched the wall", or "some of your hair didn't go in". May Esther say, "No, I'm 100% sure you're wrong"?