[Amazing Yorkies]

I appreciate your concern!
Besides the typical behavior change in his personality after breeding that you may not appreciate.. all breeding Yorkies should be health tested for genetic health problems, as well as knees and hips checked. You wouldn't want to pass on an unhealthy trait to puppies.
Study the YTCA Standard so that you know that the Sire and Dam
at the very least, have the appearance of what a Yorkie was meant to look like. So many people come on here wondering if their Dog is even a Yorkie. By breeding to Standard, there is a much better chance of a Yorkie actually looking like a Yorkie.
You should also know the background of both Sire and Dam to make sure that you are not doubling up on faults or traits in a line that could be passed on to the puppies.
Breeding is so much more than putting two Yorkies together for cute puppies. With breeding comes a responsibility to produce the very best, healthiest puppies that you are capable of... even for pets.
You might find out from the breeder what her intentions are, and that she is doing the right thing for the breed and her future puppies. If she isn't, she'll probably breed anyway.. but you will not have contributed to the problem. IMO