Your friend has not dieted before, (I presume). Every time you have dieted, you have caused your metabolism to reduce the number of calories it burns at rest. This is what I have read, anyway.
Once you have dieted, you need to stick with a healthy way of eating permanently, or you will have a tendency to gain weight that a person who has not dieted will not have.
Yo Yo dieting will cause you to have a propensity towards weight gain.
Exercise, however, is great. It will give you muscles that will make your body look better even if you are overweight.