Well, I did something I never thought I'd do. It took some thought, because it would probably make me come off as an asshole, but there really was no way around it.
I prematurely stopped an RFDR of someone.
The simple fact was that there was too much wrong with the manuscript. The narrative meandered all over the prairie. It threw character after character after character after character at you. And I never understood who the protagonist was, what her goal was, what was at stake, what she needed to do to accomplish her goal, or who was trying to stop her. Those are fundamental problems that have to be fixed before I can start talking about plot development, characterization, pacing, and overall reader experience. No matter how much you polish a turd...
So I stopped reading. My primary reason was that I'd already said enough about the manuscript. Any more and I'd be repeating myself. The excess would not mean the author had learned anything new, and I myself wouldn't be learning anything either. Reading the novel was arduous - I knew this thing wasn't going to be any good unless it was rethought and rewritten. Why say in a hundred words what I can say in three? (I think there's a quote to go along with that, but I'm too lazy to look it up).
So I sent him a polite, professional letter, explaining my reasons and actions. I said I didn't need credit, I just wanted him to use my advice. I hope he understands, and doesn't report me. We're all in this game to improve, to better our writing. I hope I did that.