Mid section workout

I have a client who is 125 lbs with 25% body fat, she is toned and lean except for her mid section. We do Pilates ab series and other Pilates excercises, ab machines, crunches, full sit ups with weights, and we've gotten into hoola hooping, still this is where most of her problem area is. Are there any different things we can do? Or some dietary change that may help?