If you feel her feelings would be hurt by coming right out and saying she should not be teaching yet, then I would say something like this: “As you know, good technique is very important, and it takes a lot of experience to recognize bad habits and correct them before they become extremely difficult to eradicate. So I suggest that your student come to me at least once, preferably twice a month, so that I can see if there is anything requiring correction. This would also help you in designing a lesson plan. If you prefer, you could both come and we can work together.” The student’s student should keep a notebook into which you both would write the week’s goals, exercises and pieces, with all the suggestions in point form. It would not be long before the student would recognize the better teacher. Even if you only gave her a lesson a month, the girl would at least have a chance to become a proficient and uninjured harpist.