Have to agree with Historymystery. Years ago, when I was in my early twenties and not very assertive, I was asked by a colleague to read through a novel she had written. It was a really difficult experience, because the book was about a child that had gone through emotional abuse at the hands of her mother and then her life story. Not only was it very depressing reading (and not particularly well written) but I also started to realise that it was autobiographical and even recognised some of my colleagues in it. She essentially wanted me to proof read it, and give her an opinion. To be honest, there were so many errors in the first few pages that it would have been easier to focus on which bits to keep, rather than which ones to cross out or correct.
Come to think of it, this woman was quite similar to a certain other author. If anyone criticised her or disagreed with her, it was because they were jealous of her, deluded or both. I quickly made an excuse ( I was working full-time and studying a post-grad course at that time and had a two year old, so hands quite full) and returned the manuscript.