I really hate to say this, but writing in the style of an author is not a crime. If the AI plagiarised, then they have a legitimate case. You can even use the characters created by another author IF THEY ARE NOT TRADEMARKED and all you have to do is say the characters were created by them in the preface. You dont even need to pay. However, since both these authors have hit the screen, the characters would be copyrighted. So they have a case. The issue then becomes if the AI generated work was technically published by OpenAI or by the user of the AI. Be interesting.