I agree with everyone's comments.
This goes to the heart of the copyright laws and their issues. It is also shocking to hear from someone who works directly in the industry.
I would not continue the practice (if you have in the past) of using the store's light box in the future even in a class setting. I would get my own to use at home.
I would have a frank discussion with this person. You might also want to have this discussion with others being present. That way you would have witnesses to your conversation with her. You are right in your opinion and stand. I would explain your thoughts to her and say that as a professional herself, you know that she will not take your answer personally. She should want to support the designer and her work and the buying of patterns and books is how that is accomplished. She needs to get her own copy for her own use. If she does hold this against you in your future dealings then she was not a true friend.
Pam M