I think the main thing for my parents was the alcohol. They're by no means problem drinkers or anything, but they do like to drink on the weekends as well as with dinner. I think that's why for them the Dukan diet really couldn't be sustained as a lifetime type of thing.
Also, mum is an amazing cook and loves to make all sorts of things...so she was pretty limited by that plan.
I know it's not the same thing, but my friend from work did Atkins years ago and has still maintained her weight since losing over 25kg. She had to be fairly strict whilst on the plan, but these days she sounds similar to your friend and eats what she wants to but is strict with portions. I do think she has 'carb free' days as well, still.