Ask for a phone number for the pianist, and have a conversation with her. You should be able to figure out rather quickly if jazz is in her wheelhouse, or not. Some "legit" musicians (myself included) might not sell themselves as jazz players, but can hold down the fort when needed.
If she is exclusively a classical player/needs written charts/isn't comfortable with the real book, ask her to also contact the administration and express this. She likely doesn't want to look like a fool any more than you do, and should also know the amount of time needed to prepare an hour of background music isn't worth it. Although I have met some rather stubborn administrations in the past, they should respect your desire to uphold a level of professionalism as a representative of their institution. If they don't respect your professionalism, or your profession, they why did they hire you in the first place?