Every waxing place I've been to gave me the fifth degree about being on prescription acne medication*. Frankly, I'd be concerned that they didn't ask your niece, and wonder about their procedures and if I should go there at all.
*Interestingly, I don't think this screening is done in non-acne aged people, as my mother is on retin-A for something else, and has never been screened when being waxed.