Why is my dog afraid of my best friend? Shes fine with everyone else. And shes known my friend since she was a pup. This behavior is new.
I would guess that either your friend is spending time around another animal that your dog doesn't like the smell of, or your friend is being mean to your dog when you're not around.
It could be because that she is still not used to your friend, even though she has known her since she was a puppy. The best way for her to get used to your friend is by your friend spending more time with her, and maybe give her some dog treats.
RastedBlemnants has the only answer given the limited information. Or.....I could use YOUR answer to another Q (almost the same as this Q)..... "Your friend is a demon"