I feel like they should try the potato diet. I recommend it, not because I believe it is effective (it wasn't for me), but because it is very good at removing the pleasure from eating. When you are eating to survive, you will come to understand how what being sated is. From there, I found I could lose weight because I had broken the habit of eating for any other reason than to fuel myself. Although, admittedly, I enjoyed losing weight this way better than the potato diet.
With some people, I think they have just lost what being sated feels like.