I have been both a published author and a book reviewer, so I think I am qualified to reply. As a reviewer, it is quite natural that you'll like one book and not like the other. And a negative review makes sense since you gave that a precious few hours of yours to feel disappointed in the end. But at the same time, a review is something that will show up everytime someone searches for the book and compared to your few hours, the author had put maybe a few months in that book. So in my view, even a negative review should also focus on the good points of the book. Specially if the author sent you the book himself, you can imagine it would have costed him some money. If the review is your own i.e., not because the author asked you to put it, that does give you all the right to be as critical as you want, but again, a bit of a pros and cons type review is always preferred.
There will be times when you absolutely hated the book and can't find even a single good point. I'd say go for it only if you bought the book yourself. If the author had sent it, it is always better to discuss it with him before publishing it. It is not binding on you to take the review down, but you need to think from the author's perspective too and to me, it does sound a better option to talk first.
Completely agreed with the opinion above "Ask him to point what problem he has, and if he has an opinion, add that opinion in the end of the article. Let people know that. If they agree to what the author said- fair enough."
To end it, I've had experiences where I was asked to take a review down. Except one time, I didn't. I did change the language a bit without changing the crux of the review though. I was lucky enough not to have received a very bad review for my own book. But one time I did get a call from a reviewer who told me that his review wasn't good and if he should post it or not. I told him to go for it, but I loved the fact he cared to contact me first.