We haven't been to DL yet, but have booked some priority seatings for our upcoming Thanksgiving Trip and I inquired about GF (as well as dairy, soy and nut free) offerings for my DS, who is on a restricted diet due to food allergies. They made a note of my son's allergies on our reservations, and said that the chef will come to our table to consult with us about his choices. I'd suggest that if your SIL has any table service reservations that she do the same, it always helps the chefs to know ahead of time (I think it helps them managing their kitchen to have a heads up of special requests, plus gets us the 'royal treatment' in service.) In fact, during our last WDW trip, we had a table service reservation at a pasta place and they had the GF pasta ready to go as soon as we got there!!
We've travelled a fair amount since DS has been on a restricted diet, and in our experience, Disney is by far one of the most accomodating destinations for special diets--the chefs and kitchen staff are well-trained in avoiding cross-contamination, they really bend over backwards to have tasty substitutes for gluten items, and we've never had a negative experience. I hope your SIL has a great trip
