Many times I've been asked to review another author's book. I always feel we should all help a fellow writer in the craft of writing. To give my honest opinion, but giving a negative opinion. No thanks! Personally, I believe that's an infliction that no writer should have to suffer from another writer. There are plenty of critics out there be a professional or non-professional. There are books that I have turned down or past on because that's not the kind of footprint I want to leave.
As a writer, it can be very treacherous to give everyone your ear. There are so many obstacles that we're trying to overcome in order to just write on a daily basis. It can sabotage our creativity. Keeping our head in the game and tuning out the distraction is all too real. As your mother has always said, "if you can't say anything nice, then not to say anything at all". There are plenty of people all too willing to give you their opinion bad or good. My thoughts, should you ever critique a writer's work you better choose your words carefully. You might be the one eating them later. We bleed ink and emotions, cleverly crafted words that make their way to a page, a book, and imaginations. We open minds with just ink and a page.
We all get by with a little help from our friends in the end.