I think one thing I'd be interested to know is why she took her Mum's death out of the first one. We know it didn't fit the real timeline, so it was right to be taken out. So that's fine:
If I'm right she initially put it in the first book, took it out and had it in a second. In which case:
Did she realise the date could be checked and remove it, or did the agent or publisher say take it out?
Because if the agent or publisher told her to take it out, then either they knew it was wrong in the timeline, in which case they should have been alerted and be doing more thorough checks.
Or they should have been alerted later when she added it to another book.
I would say that whoever took it out does have a case to answer.