Not when you are morbidly obese like the author was. When you are that big( pushing 300lbs) it's not uncommon to loose 5lbs a week at the start.

I dropped 40 lbs in 5 weeks when I changed my diet to solely fruits and vegetables (no gluten or dairy). Dropped from 185 to 145 with no exercise. The point wasn't to lose weight but to eliminate most of the possible food allergies (and later slowly add them back, to figure out what you're allergic to), and as a side effect I lost 40 lbs. I felt perfectly healthy and I've managed to stick around 150 lbs since then, after switching back to a mostly normal diet (minus gluten). No idea why "2 lbs a week" would be the "safe" rate.

The average weight loss is supposedly that. Not everyone falls into that range and so long as you are eating enough to live, are enjoying yourself and not starving to death and are getting exercise it's not dangerous.

That said, he (and anyone doing it) should probably check in with the doc just to make sure something internal isn't getting out of whack.