Welcome to Yorkie Talk! There are many possible explanations. Does your boyfriend live with you? If not, maybe she is just trying to help alert you that he has arrived, plus there is the novelty of someone she doesn't live with. If he does live with you, does he have a more regular schedule of coming and going than you do? Yorkies are really good at telling time and distinguishing car noises.
My boys are equally bonded to me and their grandpa who lives with us, but they bark more and are more animated when dad comes home than when I do.
