I did the Atkins diet...that was the worst diet ever! I got severe constipation and my hair would fall out in clumps. Definately not enough vitamins or fiber in that diet. Just like your friend, I stopped the diet and then gained A LOT of weight. It wasn't like I was pigging out, it was just that my body was wired differently at that point (used to high fat and low carbs), so it took time for my body to adjust.
The reason why the Atkins diet is no longer popular is because it is so restrictive, and because of the LARGE weight gain after either stopping the program, or "cheating" and having some carbs. You should tell your friend that you need carbs (the good ones, that is) for energy. Not to mention carbs clean out your system (all the fiber). I would tell her to slowly add the carbs back, so that it doesn't put her body into such shock. Also, let her know that she is a lot healthier now, and the food pyramid is there for a reason
. One last thing, Dr. Atkins had heart disease and some other really bad health problems when he died (I can't remember, but I will look it up and get back to you on that one), so it obviously wasn't a good plan after all!!!