Fear or Protective behavior ? You can stop this by letting her bark 3 alert barks, then give her the cue word to stop. We say 'Enough' . Go back to basics like sit, down, and stay. Remind her that you are the parent not her. Be the PLP ( provider, leader and protector) If she is 6 months old you can give the cue Enough and if she keeps barking, spray her with water. ( quick short bursts dont drench her, you are simply breaking that bark trance.) Praise with good quiet, if she gets quiet. check out Nothing In Life is Free training, if you think she is just being bossy and bratty.
Now if this is the only person she does this with, I would trust her instinct. A brief story to show my point. My wonderful Willbe (RIP) was a wonderful dog. He was spunky but not aggressive. He went to work with me every day. He stayed in a basket under my desk unless I gave him the ok to greet someone. One day a customer came in to pay his bill. Willbe went crazy dancing and barking. I shushed him and told him to get back in his bed. He obeyed, but never took his eyes off the man, and kept growling in his throat. I was embarrassed and told the man I was sorry. After the man left I felt puzzled. He came out and sniffed where the man had stood and went to the door and looked out. The following month the same thing happened. The handsome youngish man smiled at me and said, "it is ok, maybe he knows I am not a dog person." A few days later the handsome youngish man had been arrested for several rapes and a possible rape murder !!!!" I have never forgotten that lesson, if a dog does something out of their normal behavior, pay attention !
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