I cannot imagine an Hermes SA voicing an opinion about the authenticity of a bag being carried by a customer unless the customer asked to bring it into the spa. Even then, the SA's often take the bag and then later have someone call the customer and ask her to pick up the bag and tell her it was refused due to lack of authenticity. No SA is going to say, "Madame, you do realize that you're carrying a fake, don't you?"!!! I have an "inspired-by" Constance and have often been carrying it when I am in the boutique (including FSH), and no one has ever commented on its lack of authenticity.
Call me a cynical old fogey, but I smell a hint in the air of "I want to take a bit of the wind out of her oh-so-proud sails by informing her that her bag is a fake". Frankly, it does not matter if it is or not. It's not against the law to CARRY a fake bag - just to sell one or import them.
Unless she expresses concerns about its authenticity and asks for your advice, I'd keep my mouth tightly closed. I still say I'm astounded that she would truly believe that a matte croc birkin brought back from a trip by a BF, NOT a fiance, would be authentic. Do BF's regularly these days buy bags that cost $30,000 as a bring-home-from-a-trip present? If so, I clearly need to find a new set of friends!