I don't think you have to explain or "talk around" the problem -- just state your policy as tactfully as possibly.
I do like the idea mentioned of offering to have the lady bring her friend in to meet you and ask any questions beforehand. That offer shows that you really do care about the comfort of the new student and allows you the opportunity to start developing a rapport with her. The lady can get her enjoyment from being the "big sister" in the situation, showing the student around.