However, I don’t know how to write an unassailable contract that would hold the proofreader “free from harm” for assisting Dijana in a “tell all”,
Not difficult at all. (If I did such a thing I'd have my attorney review it). The contract would cover the scope of work, which would be correcting spelling, grammar etc. Somebody else is responsible for double checking the contents of the story.
Although if its written as a memoir you can probably cover your butt by claiming that you're written about events from your personal perspective. That's hard to prove wrong in court.