If your interest is weight loss as a means to get faster, I suggest you find a qualified nutritionist or trainer who is into cycling. If you are riding to lose weight, I suspect a common sense application of the Zone may be OK. As your friend shows however, a "weird" diet will yield only fluctuation, not real loss or increased fitness.
I noticed you are in the Dallas area. I ride with a guy who has whipped a ton (literally!) of people into fast riding shape with nutrition and structured bike-centric workouts. I am looking to create the time so I can do that too! Private message me and I'll get you the info if you want it.