I agree with the other poster, I would take this little dog to your vet and explain what is going on. Make up an excuse as to why. If this girl is starving the dog to keep it tiny she needs to have the dog taken away. Tell the vet what is going on. And the vet should keep the dog and report the abuse and also make the owner come in herself to pick up the dog. That way the vet can tell this girl what she is doing wrong. Does she even check on the dog when she drops it off for a week or so? It sounds like she wants all the benefits of having a Yorkie {the attention} and none of the responsibility. I can not believe someone is so immature and stupid as to think that by starving a dog they will keep it small!
Please keep us updated.
I am tempted to tell you to lie and say the dog ran away and keep the dog yourself but I know that is not right. Call animal abuse