A good editor works with the author’s voice; they don’t impose their own. I’m fond of comparing editors to chameleons because good editors take on the colours of the manuscript they are working with.
I’ve been teaching editing at the University of Canberra for many years, and during that time I’ve observed that the majority of aspiring editors arrive thinking that editing is directive. That they have to identify everything that is ‘wrong’ with a manuscript and tell the writer how to fix it. This usually involves ripping it apart and putting it back together again in the way that they themselves would write it, since our own idiom is what sounds ‘right’ to us. Part of becoming an editor is learning how to identify and curtail our own idiom, so that the work doesn’t have our fingerprints all over it. It’s important to retain the integrity of the work and edit it on its own terms.
Irma Gold on editing.
This goes for beta reading–including fanfic beta reading–not just “pro” level editing.