Well, I think the easiest way would be for both of you to be online and on the phone at the same time and you enter in all your information on the Disney website, make sure you're signed in, and put in the date and time you want for that restaurant, and the minute she cancels her reservation, you would hit the button to search for the times you want, and ideally, that time would now pop up.
Or, I guess, she could keep her reservation and give you her name and confirmation # to use? Is that allowed? I'm not sure. Does Disney care that you're signing another name when you pay the bill? I would think they wouldn't care. It's not like they check photo ID when you sign into a restaurant.