He was thinking of stopping his current series and starting a new one, all based on the comments in the reviews. My advice to him: DON'T.
1) Not every book will appeal to every reader. Sad, but true: your book, no matter how much you worked on it, no matter how much of your heart and soul you put into it, no matter how much you worked, edited, re-worked, and edited some more will still not be for everyone. Think of the classics, traditional or new: The Odyssey, Shakespeare, Moby Dick, Anne of Green Gables, Pride and Prejudice, Stephen King, and more - they have reviews and ratings that run the gamut.
2) The strength and impact of the review matters. Does the review say something to the effect of: I hate (your genre) and this book sucked? Did the review actually point out inconsistencies, grammar issues (heavens, no!), provide issues that you could logically discuss? Or was it basically a rant, offering a personal opinion? There's practically nothing you can do to change a person's opinion about something, so why waste your time stewing over it?
3) Some people are mean. You know who they are in your real life, and how to
4) Realize that readers make up their own minds. Personally, I don't put much stock in any review online. I'm a little judge-y on how the review itself is written (please, dear God, save me from reviews written in all caps or in text-speak!). I look at the overall numbers and if there are more one- and two-star reviews than the others, I'll take a peek at what those say. I had another author share that she particularly appreciated a one-star review from someone who NEVER READ THE BOOK (insert your own sarcastic tone here). Reviews like that get tossed out of my mind and almost have the opposite effect - I'll read the book just to prove them wrong!
I'd like to leave you with this final thought: as an author, you have worked hard to put out a piece of art. Art is subjective; there will be no two people who get the exact same thing out of it. But that's ok. You didn't really do it for them, did you? You did it to get rid of those voices in your head (really, once they start telling you what to have for breakfast, that's when you should see the head-shrinker!); you did it to explore an issue; you did it to create a new world and answer the question what if? And you should be proud of it. After all, not everyone has the talent and drive to do that (me included!)