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.
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.