My understanding is that everyone in your party must buy the plan, no matter how much/how little each person will eat. You might check on the disABILITIES board here
http://www.disboards.com/forumdisplay.php?f=20 to see if anyone else has experience with this.
For any character meal/buffet/all you care to eat/show you mom would attend with the rest of the group, she would have to pay the same price as any other adult, because it's considered that you are paying the price for the experience/show/character interaction etc, not just the food.
Here are some ideas -- and please no flames

because I don't know if these would even work, but maybe someone else will have a definite answer. I also agree with patsal -- contact Guest Services or the coordinator of your Grand Gathering. If you are all together as a group, and everyone has paid for the
DDP, then maybe your mom could just not order a meal on the DDP -- maybe one of the members of the group could give her their appetizer instead, or she could just order something small OOP. I'm not talking about the set price meals I mentioned (buffets, etc) but regular TS restaurants. Then with her remaining TS credits, you could probably cover part of a meal for the rest of the group, depending on how many days you are there.
Or if there is a kid in the group on the child's plan, but they want more food, or would prefer selections from the adult menu, maybe they could swap entrees with her. Or your mom could order her full TS meal, just eat the appetizer and beverage, and give the rest to the child. If she is really worried about the cost for the plan, and everyone else really wants the plan, maybe she could pay half -- $20 a day -- and the other adults in the group could chip in to cover the other half of her cost.