Renata and Her Husband Split
In the book, there isn't really a Monterey Five. Renata moves from Australia to London by the end of the novel upon discovering that her husband had an affair with their nanny. There's no affair in the series, but their marriage has seen rough days with Amabella's school situation.
Since we want to keep seeing Laura Dern's story, it's possible that Renata may leave her husband and seek support from her newfound (sort of) pals. We haven't seen a clear motivation for divorce, but the book does provide potent ground for one. The Hollywood Reporter says that Gordon "will face the consequences of a big mistake and risks losing everything along the way." This could be a description for the fallout of an affair, no?