I didn’t think about that part (apparently my reading comprehension is bad enough that I also used the wrong pronoun for the author). Assuming Main actually saved the writing samples and does the work to match them all against the author’s novels, then she should be able to pin it down. It will be interesting if we get to see what damages she sues for.
If she is willing to lie she could still say somebody else signed the nda and forwarded her the information to post. I wouldn’t do that personally, but I also wouldn’t think unkindly of somebody who did.
