Perhaps she was with a “westerner” and so the shopkeeper assumed that she was whitewashed and did not speak a lick of Chinese.
Living in manhattan, you will find that a lot of people that speak languages that you would not imagine that they would. I am fluent in Cantonese, Mandarin, Japanese, and French. Not long a go, a Taiwanese friend and I were busy chatting away in French in a Chinese restaurant and said something catty about a neighboring table. The other Asian couple responded back in French and asked him why he was so rude. So one has to be truly discreet. Never know who might know/understand what you are saying about them.