how long was your friend working out for? Was it daily?
I agree that fasting can be very beneficial for health but everyone likely has their own threshold of how long it is safe to fast for. You most definitely don't want to get down to the point where your body is attacking your organs for energy or amino acids.
There are essential amino acids that your body needs that it can't make on its own.
RE: I Quit Eating for 28 Days [Water Fast] : Day 8 (Focus)