IMHO I agree that this is unacceptable however I was actually taught to help your buddy on with their gear so this likely falls within limits
This should have been an oops moment where she ate her words and displayed he other skills and went to retrieve the mask herself. Obviously if the other instructor was there to retrieve it for her, she would not have been leaving any students alone.
IMHO, you do not yet know what it takes to be a safe instructor other than an uneducated but possibly very intelligent thought process. Watch out how you label someone as unsafe (or incompetent) and simply decide you want to switch because you are not comfortable with that instructor.
What should you do??? Since you are part way through you may pull the other instructor aside and let them know you would be more comfortable with them rather than the other woman....but ice. Tell them it is her teaching style or something rather than get into a fight about weight. Finish your training and get your C Card, then pull the LDS owner aside and tell them your concerns. Perhaps that person will be talked to....perhaps not.
If this does not work and you cannot get the other instructor to take you over and still feel unsafe, stop your training and find another instructor elsewhere.