If you start rapidly losing (or gaining) weight with no obvious explanation that is a cause for concern. If you start eating much less, and exercising much more, you probably don't need to ask a doctor why you're losing weight.
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.
