Unfortunately, anyone can call themselves a pet behaviorist, or a "breeder" for that matter. She may know a lot about training, but she doesn't understand specific breed's need. Most dogs are ready to leave their moms at that age, but important socialization takes place between mom's and puppies, including inhibition of the biting reflex. I would not buy from a breeder who didn't understand this, she has not read up on the specific needs of a yorkie, and this means she may have not up on health problems of the yorkie and not done any of the required testing of the breeding pair. This Maltese site explains more about why small breeds need to stay with mama longer. Maybe you can even teach her something.
Why do Maltese puppies need to be 12 weeks old before going to a new home??
Taken from the YTCA website:
If you are seeking a family pet a reputable Yorkshire Terrier Breeder will not release a puppy to a new home before it is twelve (12) weeks old. This ensures the puppy is old enough to have had some of its shots, has been checked by a veterinarian, and a health certificate issued. The puppy’s ears should be erect, and you can generally tell what size the puppy will be at maturity when it is twelve weeks old by doubling the weight. Although not a definite rule it is generally thought that if a twelve week old Yorkie puppy is three pounds it will likely be six pounds at maturity. Also, by twelve weeks the puppy should be outgoing and well socialized so it will easily accept the transition of leaving its mother and siblings.
http://www.ytca.org/faq.html#D