While my legal education does not extend past Perry Mason and Judge Judy, please don't take this as legal advice. But I feel compelled to give my opinion as I had a similar experience with H and W shareholders and it makes me angry seeing someone try to take advantage of a friend for their own gain, which is what Books is trying to (blaming you for his carelessness, ignorance, and greed).
In my case I had all hard copies and notes, but as you say you had to make adjustments so wouldn't you be able to retrieve hard copies of the returns and amended returns if prepared? From your synopsis it appears your memory is good so why not re create notes, and explanations, especially the reason for Pro not being able to deduct losses, from your knowledge of what Books was telling you and the continuous statements from Books of unreported loans and other items. I don't think this is a violation of 230 as they are both questioning and answering the same situation, I don't see it as talking about one without the other's permission.
I think your being prepared will show them they are barking up the wrong tree.. Getting a consultation from an attorney and speaking with your E&O will give you good preparation as to what you might expect and what you should do.
Count me in for the drive by if needed, and let me know if you need me to call Brooklyn. It's not you, it's them.
Good luck,
Bill