Tell her this:
Our body stores fat for the emergency purpose. When you starve, the tendency of the body to store fat increases, in anticipation of the emergency. Your metabolism goes down. So when you will start having normal diet, you will regain more than you lost, because your metabolism has gone lower than it used to be.
On the other hand, if you are giving regular foods to your body, it will not have a tendency to store fat for any emergency purpose. Your metabolism stays high.
The right way to lose weight is to never starve your body!!!