Years ago when I was a semi-young punk flying around on big radar planes, I had a buddy in my office give me something to review. I pulled out my red pen and bled all over the sheet of paper he’d given me. He looked at the work I’d done and said “You didn’t fix my errors, you changed what I wrote into what you would have written.”
Oops. He was right.
I’ve done a whole lot of writing and a fair amount of editing since then, but that incident has always stuck in my head. Don’t waste your time during editing by needlessly changing the author’s voice, style, or approach. If there’s a gramatic problem, edit. If there’s a clarity problem, edit or point it out. If there’s a structural issue, edit or point it out. If there’s a difference in style or voice from how you would have written it, leave it alone. You’re editing, not writing.
(I’m not complaining about the editor on our book, BTW. He’s done great work for us and has helped me avoid looking too much the fool.)