The two main reasons are behavior and immunity
staying longer with the litter allows the puppy to learn ranking order and social skills and experience corrections from the mother. It is also pliable to bond to humans rather than dogs.
If the pup is taken away early, it hasn't yet learned how to interact with other dogs and may respond aggressively in the future to the presence of other dogs...also, bite inhibition is usually taught through littermates...biting, wrestling, playing..etc..if you take the pup away from the litter too early, it won't learn all of this...and trust me, it causes problems..I know from experience...I own a puppy mill dog that has been taken away from its litter mates at an early age...not a good thing...
Also, the pup hasn't learned how to respect corrections and a lower rank from its mother...this will also most likely cause dominance problems in the future..again, NOT a good thing..and something I'm also having problems with...
again, I DO own a puppy mill dog...they're ALL taken away from their mothers too early
unfortunately..I wasn't educated about puppy mills back then and when I was a kid my parents bought me the dog from a pet shop...thankfully I was the first to find out and educate myself on all the junk that happens "behind the scenes" and informed my parents..now we solely rescue!..however, my chihuahua is currently 8 years old and has alot of mental and behavioral problems that are very very difficult to control at times...all more than likely due to the very reason we're talking in this thread...but anyways...
If I were you, I would strongly try to convince your friend that she should return the dog to the so called "breeder"...I would rather label them a backyard breeder...
if she returns the dog, it will let the breeder know that people aren't completely uneducated about the situation, and she should furthermore give the breeder alot of heck ....on MY behalf lol...
Let her know that its best not to support people like that...if nobody bought from these buttheads, there wouldn't be backyard breeders or puppy mills.....and a heck of alot of unwanted animals
good luck..and please let us know what happened!