I would just pay the difference and be done with it. You need to look at the dining plans for the rest of the trip. If she is on one level and you are on the other, do you all eat together or is she off on her own? DOes she miss meals? Does she need to skimp if she joins you?
WHen we plan extended family vacations, not just WDW vacations, we plan what everyone can afford, and plan to the lowest economic factor. If DH and I choose a more expensive resort, meal, etc than the others are comfortable paying for, we pay. IMO, when you are vacationing and have pushed one persons budget to the edge, dining is the area where the ddifferences are most evident. People can manage their portion of the resort. They can avoid shopping and specialty snacks and beverages gracefully. But meals bring the crew together. That's when the rubber meets the road.
My Mom's funds were limited so when she traveled anywhere with my sister or me we picked up all of the meals. Every one. (Except the time we went to Atlantic City for a long weekend and the only one who had money left was my Mom. She had to buy our coffee on the way home) This let her order what she wanted because she knew that it was our turn to treat her for all of the times she treated us.
Anyway, my point is that if Mom cannot afford the plan you are on, and you cannot ay the difference, I would drop it to whatever is in her budget and make that one work for you. The best way to make someone feel left out is to be left behind for meals.
