Sigh. Ugh. This is painful and very frustrating. Honestly, even with that kind of a condescending dumb a$$ comment (and how on earth could she say that without even asking your dogs age?!?) I honestly can only imagine she would choose to approach you to give you (unwanted and innacurate) health care information because she thought she was sharing something that would help your dog and is just too immature, uneducated and inexperienced to know appropriate communication techniques (seems like a lot of 'grown ups' lack that too) like saying "hello!" With a friendly smile and asking some questions about how to best serve you and your dog first off and secondly to either have information or aknowledge where she is lacking some and when it is best to step back.
My heart is soft on this issue for many reasons- first off that I work retail in the pet and livestock industry and have found myself vaccinating dogs and deworming goats in the parking lot, "gosh mam, maybe your goat is sick because you keep feeding it garbage sacks of white bread from the discount store and goats actually can't eat that..."
And also because I have two young, beautiful daughters aged eleven and thirteen who will surely in a matter of short years be ditzy sales representatives at some low paying job where I hope they will never be presumptuous or poorly spoken but in the instance that they are I hope so very much that they are corrected and put in their place but in a way that they can learn and grow. I thinking is appropriate and necessary to address misinformation- it can be very dangerous. A letter to managment might be a very appropriate way to do that, sometimes there are very helpful educational seminars that employers can take advantage of and help educate their employees- maybe this silly and offensive young gal could get some good developmental opportunities out of it