I'm going the week of Thanksgiving, not Christmas, but that's been a tough week to find openings as well, so perhaps my experience will help you......I found that the restaurants in the resorts had much more availablity than the ones in the parks. Obviously, I'm not talking about the much discussed and very popular places like Ohana, Kona, Chef Mickey, etc....but these are the places that I found with availablity to the point that I was asking "do you have anytime on Tuesday here" and was getting "well what time do you want" and then when I'd say "something between 5 and 7pm would be great" they'd start listing at 10 minute intervals!
Cape May Cafe at Beach Club (it has a character breakfast with Goofy in the mornings.....I was amazed that there was a character breakfast anywhere available!). The dinner is a clam bake.
Garden Grove at Swan....they have a character breakfast on Saturdays and Sundays with Chip & Dale. Again, I was pretty surprised to find a character breakfast available.
Olivia's Cafe at Old Key West
Big River Grille & Brewing Works at Boardwalk
Boatwrights at Port Orleans
Grand Floridian Cafe (they didn't have breakfast the day I wanted, but we got a lunch ressie at 11am)
There are also a lot of really good CS meals available......I'm finding that not every night has to be a TS. I originally thought most of the CS were burgers and chicken fingers, but that's not true, especially in Epcot of course. At AK, Flame Tree has such great reviews that we're doing it for dinner.
Good luck to your friend! She's lucky to have you, can you imagine showing up on the busiest week of the year and having no meal plans. Even if she can't get into the popular places, at least she won't spend most of the day foraging for dinner plans with your help!