Could you tell your Mom that her food is really delicious, but you think you've been getting cross contamination despite all of her hard work? You can't quite figure out why. (Makes it less of blaming her and more of an "our problem" situation) Then, ask if you could buy some pots, pans, utensils, cooking instruments for her to use that would be devoted to allergy friendly foods. You could get some obnoxious colors so there would be no mistaking it!
You could also fib and say you went to the dr and the dr said it seems you're still getting gluten somewhere. You couldn't think of where because you have devoted cookware for allergy free cooking. The doctor said that's great, but EVERYONE who cooks for you needs to do what they can to avoid cross contaminating your food. If that's not an option s/he recommends you prepare and bring your own meals. Unfortunately, sometimes people take what doctors say more seriously than people's self reports of how they feel.