It sounds like she may have developed an allergic reaction. Just because she got by doing it for two years, does not mean she could not have developed an allergy to the product, or she has changed the face care products she has been using.
Hi, I am also in the beauty industry and have come across some eople like that. What might of happened was that maybe you did not apply enough baby powder (if you used it) on her chin before waxing. It could of also been that her skin was extra sensitive that day possobly from the way she washed her face and which products she used. You should have asked her what products she used that day or if she scrubbed her face. It's also quite possible that the wax clung on to a small piece of skin that when you went to pull off the strip it tore the skin with it. Don't worry about it too much, the worst that could happen is that she wont come to you anymore, but then again, who needs an ongoing complaining customer anywaly right?
Sorry, forgot to mention treatment. Next time you see her tell her to use Poysporin.