It is great that she is a nice person but hard to tell much from the website. It appears some very small yorkies are being bred looking through the adult females. YTCA guidelines say not to breed a female less than 5 pounds. It is just too dangerous to the mother and can produce unhealthy weights in pups.
There is no guarantee listed on the website. That can tell you so much about the breeding program. A good guarantee will not make you trade in your beloved puppy if a genetic health problem comes up, instead, they will at least help pay the vet bills. A good guarantee will give you ample time to get the puppy to your own vet for exam and cover problems that might show up in that exam (kennel cough, parasites, etc). Genetic conditions should be covered a min. of one year. Ask specifics i.e. what will she do if you have the BAT test done at 16 weeks and the liver levels are sky high?
You should get to see the dam and sire of the puppies you are looking at. You need to see that they are healthy, of good temperment, and meet yorkie standards. Ask to see their pedigrees. Ask what pre-mating testing was done on the dogs. Ask about other litters -- health problems? Stillborns? Any LPs, Liver problems, etc?
Find out when she lets her puppies go to new homes -- should not be before 12 weeks. What vaccines will they have done at that time? Most importantly meet in person and see her with her dogs. You can tell a lot by how they treat their dogs and puppies. Of course, all breeders are on best behavior in front of you, but you can usually get a good feel for how they are with the dogs and in turn how the dogs act to them. See the place where adult dogs are kept. Too often, larger scale breeders want to just show the puppies in the house while adult dogs are caged in a separate place (sometimes not too nice). You are first deciding on the breeder, then the parents, and then the puppy! Remember all the puppies are going to be adorable! You have to know the other terms are met before you ever pick up the puppy. I have to make myself walk away and not hold a puppy if I don't like what I see because I have like a disease -- once I hold them I have to buy them.
The personal references are a good start but look further. This is a lifetime commitment, so make sure.