Books about BPD are written as coping guides for people who don't have it because they're the ones who really need information about how to handle the situation. Borderline people are emotional black holes who suck the love and joy and comfort out of everything they touch in an insatiable quest to fill the infinite emptiness inside of themselves. They're the closest thing the real world has to vampires, except instead of eating blood they eat happiness. Dealing with someone with Borderline Personality Disorder is a burden, and for anyone who's regularly forced to, buying a field guide to avoiding the meltdown landmines is probably the best thing they could spend their money on.
What does she want the book to be about? If you wrote a book that was nothing but advice for people with BPD, it'd be like that scene from the Shining, except instead of "All work and no play" it'd just be 500 pages of "Stop constantly being an asshole, asshole."