But when it's someone more famous, it's harder to avoid. There have been public feuds between writers through the ages. Gore Vidal and Norman Mailer. Ernest Hemmingway and F. Scott Fitzgerald. Stephen King and far too many authors to list here.
In fact, Christoph Paul wrote a guide on how to start a literary feud.
While Mr. Paul suggested ripping the other writer on a podcast, the trend among indies today seems to be deliberately attempting to start a feud by leaving a horrible review on a rival in the same genre. And personally, I believe it's done out of jealousy with the justification of the First Amendment and/or a piss-poor attempt to gain attention, aka any publicity is good publicity.
Which goes back to my original advice regarding reviews on your books--ignore them.
Feuds occasionally happen over innocent misunderstandings, but you’ll have a better success rate with willful misunderstandings. - Bill Ferris
Even if you cannot ignore your reviews, DO NOT ENGAGE! Seriously, that's exactly what some of these bad reviewers want.
Sometimes, for whatever reason, you cannot disengage from the bad reviewer (like family or good friends). Because of course, they're only trying to help you be the best writer ever!
That's when I smile and say, "Thank you for your feedback! Which paragraph/sentence prompted your insight?" And then pursue that help with specifics until the friend/family member runs in the opposite direction.
Worrying about and/or dealing with someone else's jealousy is a waste of your time and resources. Focus on your writing, and ignore the idiots.
And if all else fails, sue 'em.