Wizard do you think Maggie's biting behavior is related to establishing dominance in her territory so whenever a new person or animal enters she bites them or attempts to bite them to establish this?
As you said its not all people, so Maggie must have a sense of who she feels Dominant to and who she might not.
If this is the case, then from what I understand you have to find a way to establish a higher status in Maggies eyes. One way, if you can stand it is to personally run this dog through some obedience training where she has to earn pets, treats, and rewards from you. In that way you will be establishing a bond with Maggie as well as a hierarchy in the relationship you have with her. Of course your friend her caretaker will also have to be on board with this.
I found this article on the web and felt it might be useful: