Graduate school admission is based on grades, which are frankly inflated at a lot of supposedly very good schools so nobody's kid has to suffer with a 2.1 GPA; and on GRE scores, which come from a multiple choice test. That's how you get grad students who can't write.

Mentor, can I suggest you stop taking this on as your problem, something for you to feel guilty about? The main thing I learned in graduate school was to stop being spoon-fed, formulate my own work, and have it meet professional standards, and these non-writers have failed to do this.

You could just have a frank talk with the lady, saying 'your writing is not at a professional level. Go get some books and take a course in technical writing and work on improving your skill.' Leave it up to her. If she gets pouty, she can leave, and you can hire a new candidate, who can hopefully pass the written interview questions you could use to weed out these types.