There's no such thing as paperwork proving a dog is a service animal (you can get such a thing, but it's NOT a requirement), and what you asked her is illegal. Your store was VERY lucky she didn't go after you guys (sounds like it wasn't a true service dog, thank goodness) b/c asking such questions is not OK under the ADA. (nor should it be, and please note that I have moved OUT of a building b/c of a woman absolutely cheating the system with big scary untrained possessive protective (of the whole building) "service dogs" that she never took with her...I researched it to see if there was anything I could do, but ultimately I'm glad the ADA is strong for the people who need it, even though there are sneaks that get by)