Gluten is one of Disney's common allergens, so if it's a simple gluten-free diet the quick service places will know which items are gluten-free. If it's more complicated (additional restrictions beyond gluten), table service is actually a better idea. A chef or allergen specialist will come to the table to discuss, or give a tour of the buffet. I brought a friend who is on a gluten-free diet to food and wine festival last year and she LOVED that all the gluten free items were marked, and the night before we had table service at Port Orleans Riverside. They were temporarily out of allergy menus (which would have been adequate for her diet), so a chef came out to confirm that the items she chose were gluten free! Disney is leading the industry in handling food allergies, I expect your family member will be amazed.