My momma taught me: If you don't have something good to say, don't say anything.
Hmm? Did your momma happen to teach you when/when not to adhere to that rule?
You've heard of the 90/10 rule? Some call it the 80/20 rule but I think 90/10 is closer to the truth. Anyway, how the rule works is 90% of the time, only 10% of the people adhere to the "don't say anything" when 90% of the people should do that 10% of the time. Now that I've put it to words, I think it should be 10% of the time 10% of the people do it when 90% of the time 90% should be adhering to "don't say anything!"
As the Riddler would say: Riddle me this. You've just read/edited a book by an acquaintance. The book was 'eh' and the edits will be notorious. S/He thinks they've written the next GAN -- Great American Novel. You, as a professional editor and published writer, look at it and realize, without too much difficulty, the Great Amerian Novel will be edited down to the Great Ameriacn NO. What are you going to do?
You've submitted enough times to know the publisher will be rejecting this manuscript faster than a speeding bullet back to the author.
You want to help with edits but the acquaintance feels the book is ready to go 'as is' since the spouse and two grown children plus one English teacher have already went over it with the proverbial fine tooth comb.
Who are the worst critics in the world? I don't mean the hardest or meanest; I mean the person who reads your manuscript and gives you false hope? Your mother, father, spouse and children. An English teacher usually makes sure the spelling is correct and the sentence structure is proper but doesn't find the trip ups such as active vs passive, incongruent story threads, etc.
Before a flame war begins, the above falls into the 90/10 rule mentioned above. There ARE those spouses, parents and children who actually do help but they fall into the 10% category. My first defense reader is my wife but she knows her limitations. Still, with her assistance, some edits get caught and corrected before I send it out for a professional review. In no way do I consider -- and I love you, dear -- her judgment final... at least in regards to my writing.
I will be honest; not brutal, in my final words to this author.
But, like the Riddler says: Riddle me this -- How would you handle this, either as the editor or the author of said piece?