To confirm...
First let me apologize for the incorrect information and poor guest service you received from my fellow Cast Member at DRC. I do believe the Cast Member was simply confused as how to properly book what your sister wanted.
YES, you most certainly can make a separate 1 night stay at any Disney Resort of your choice based on availability.
To take advantage of the Disney VISA Free Dine offer, the package arrival date must begin between 8/24/08 and 9/20/08 and be booked for at least 3 nights and up to 14 nights. You will not have access to your MYW Theme Park Ticket or your dine entitlements until your scheduled date of check-in (8/24/08 in your case) on that package.
If you need a park ticket on 8/23, your sister could book a one night package with a 1 day MYW Ticket, otherwise she simply could book a one night room-only reservation and spend the day at Downtown Disney or just enjoying the hotel amenities. Either way, if she is staying at a Disney owned and operated Resort she can book a one way transfer from the Airport to her Disney Resort on 8/23. She then would add a separate one way transfer back to the airport on her Free Dine package reservation.
If you book at the same resort, ask us to note that you have a continuing reservation and although it cannot be guaranteed, the resort will do everything possible to keep you in the same room. If you book at any other Disney Resort, Bell Services will happily move your luggage from one Resort hotel to the other free of charge.
I hope this helps. If you have any further questions, please feel free to post or PM me.