Tell your friend that if that puppy is taken from it's mother and siblings too soon, it will miss very important lessons that will effect it's personality and make training much more difficult.
Puppies learn bite inhibition from their mom and littermates between 5-8 weeks of age. People who get their puppies too young often complain about the problem they have with their puppies biting them. It can be a very hard habit for humans to break.
http://www.canismajor.com/dog/bite2.html
The Fear Imprint stage occurs between 8-11 weeks. Shipping a puppy or taking him away from his littermates and mom during this period can be very traumatic and have a lasting effect on his personality.
The Yorkshire Terrier Club rules require breeders not to sell a puppy under twelve weeks old. Tell your friend if she is dealing with a breeder who breaks this rule, she is not dealing with a reputable breeder.