Tell her its unacceptable, and that if it happens again you'll have to consider changing an instructor... thus giving her groveling oppotunity, and a firm kick up the backside. It's her loss if she continues to act in this manner, and there are many instructors out there.

My examiner was always between 5-10 minutes late, so i always tried to drag out the lesson a little bit, to make sure i got my time. instructors tend to allow time inbetween lessons for overspills and sometimes they can't help being late... but that's just above the limit..
Anyway...