I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
Updated February 25, 2019: removed well-known from my comment about ghost writing as I think ghost writers should be acknowledged by anybody who uses them.
I am not taking a stand on if the author in question did or did not commit plagiarism, either directly or indirectly. I will leave that to the courts to sort out.
I have no problem with people who hire ghost writers. However, if you hire ghost writers, it’s often pretty easy to check out if they plagiarized the work they are providing to you. It’s actually a wise move to make sure they haven’t plagiarized as the costs can be quite high as a writer might also be guilty of copyright infringement as well.
One of the areas where I draw the line on ghost writing is when a writer chooses to use ghost writers without informing readers they have done so. In some cases, it’s pretty obvious a writer has used ghost writers. Most often it’s because the style of the writer’s work changed over time and the change wasn’t related to some medical condition (stroke, TBI, etc.) or a result of an accident or injury.
Here are some links about the alleged plaiagirism:
Romance novelist Cristiane Serruya accused of plagiarism: https://www.theguardian.com/books/2019/feb/20/romantic-novelist-cristiane-serruya-accused-of-plagiarism-courtney-milan
Brazilian romance writer accused of plagiarizing from Courtney Milan, other novelists: https://www.latimes.com/books/la-et-jc-cristiane-serruya-courtney-milan-plagiarism-20190219-story.html
The Cristiane Serruya Plagiarism Scandal: https://www.plagiarismtoday.com/2019/02/20/the-cristiane-serruya-plagiarism-scandal/
Plagiarism and a Temptation for Every Writer: https://medium.com/@ryanjpelton/plagiarism-and-a-temptation-for-every-writer-703a43e9bba1
Novelist Accused of Plagiarism Blames Ghostwriter : https://bookstr.com/article/novelist-accused-of-plagiarism-blames-ghostwriter/
What to learn from the Cristiane Serruya plagiarism scandal: https://www.thefussylibrarian.com/newswire/for-authors/2019/02/21/what-to-learn-from-the-cristiane-serruya-plagiarism-scandal