Fasting is the absolute worst idea ever. Muscle tissue is much more bioavailable as a source for energy than fat. He will lose any muscle mass that he has all the while slowing his metabolism to a crawl. When he starts eating again, he will gain it back and probably more.
If he is dead set on fasting, there is a modified fast that is proven to maintain muscle tissue while melting the pounds away, but it is absolute hell. It is called PSMF (Protein Sparring Modified Fast) aka Protein Strictly Mother Fucker. Ive told DJ a bit about it in preparation.
In summary, you set protein at 1-1.5g/lbm depending on activity, some fiborous veggies, and 6 fish oil pills a day. This takes calories to roughly 1000-1300 for most people. He should plan to refeed after 8-14 days, the fatter he is the longer he can go. It is very hard to stick to. Think - Starvation. It has an impact on cognitive function, and basc motivation to live. Most people cave after a few days/hours.
The kid should just start hitting the gym and eating clean. Chances are he has an addictive personallity and will find addiction in improving himslef.