Great question. There are so many variables - the reason to fast is to be very strict for a short amount of time. No one goes on a "year long diet." You either want to eat healthfully or you don't.
I've found that fasting can jumpstart your desire to eat well, eat healthfully, etc. But I'm going back and forth because I'm on a weight loss quest, but I know I can't do it all through fasting.
The reason I am in the position to need to lose weight is that I don't have much control. So I don't want to get really good at fasting, when I know within three weeks of ending the fast, I'll be eating lots of pizza in one sitting.
So I'm using a combination of fasting and portion controlled eating to meet my goals. But if your cousin doesn't really want to do it, there's no diet in the world that will work.