I'm certain they will work with her! I am veg, but allergic to mushrooms (such a picnic!) so I couldn't eat their standard vegetarian dish. They were able to make me a dish with noodles (like spaghetti noodles, IIRC,) and vegetables. It wasn't terribly special, but it did the trick. A similar dish with rice noodles or another type of GF noodle sounds like it would work for DDIL's mom.
I think they will find that as long as they are clear, upfront and patient with the chefs, they will be taken care of quite well at all Disney restaurants. I have eaten probably close to 25 TS meals within Disney (holy cow!) since May of last year, and have had only one poor experience (safe, but stressful -- we got attitude from the chef.) This included various levels of food allergy, up to and including a table with a vegetarian with two allergies and two other people with 8ish different allergies a piece. We almost NEVER eat out at home, but at Disney, it is a pleasure and we feel very safe.