It sounds to me like maybe the dog has been there just enough times to consider it "home". My dogs would growl at someone coming into their home, but not otherwise. i wonder if some things could be done to let it know that it doesn't posses that space.
I agree with everyone, that behavior cannot be tolerated, and it's only going to get worse. It would be too bad if the dog rules got changed, but it's better that no dogs are allowed in that that someone, or one of the dogs, got hurt.