is she bipolar by any chance? if not, does she have a family history of bipolar or schizophrenia? or, does she have any cluster-b personality disorders?
The only signs we can come up with were a few times going back 20 years, either at a party (drugs) or a canoe trip (drugs) where she thought people were talking about her/plotting against her. I wasn't on the canoe trip, but I always believed her that some people in another group of friends we know were mercilessly cruel to her. Now I tend to think it was a manifestation of whatever this is.
Other than that she's been one of my most solid friends and always had her shit together, very successful career, someone I would call when I needed life advice, etc.