Did the author come up with the main ideas, character arcs or plot devices himself? Did he ever seek assistance from AI to come up with new plot points, rewrite paragraphs, create dialog?
The only thing which really matters is trust.
I would almost give that a pass as long as we could prove that the person had sat down for 10s/100s of hours to type out the novel.
In that instance it would be almost the same as false authorship and stealing credit for other people's work. This has existed for centuries already.
As to being influenced by an LLM I think that is fine, even up to entire plot structure.. as with the above this could happen between people too.
(I don't have an answer, just wondering.)