Editing A Friend's Manuscript: Suggestions or Rewrites?

A dear friend recently sent me a novel she's been working on for some time. She asked me to edit it before she begins the submission process. Not only is this person a dear friend, but she's also a fantastic writer. I'm looking forward to delving into her thriller. But as I was reading the first few lines, I realized I had to delicately balance my suggestions for improvement with my urge to rewrite certain passages.

No matter how talented or experienced a writer is, there are going to be issues---be they typos, plot holes, inconsistencies---in a 100K manuscript. I'm certain this one will be very clean, but still, I'm sure I'll find things I'd like to see improved. In addition, my friend expresses herself very differently than I do. I don't want to edit her novel to make it sound like my novel. I want it to remain HER NOVEL.

When you're editing for content, be careful not to rewrite the book. It's imperative that you realize the style differences between you and the author. It's impossible to write in the exact same style as another person. So be careful! Even if you intentions are the best, you don't want to dilute the author's voice with the editor's voice.

Suggest changes, mark passages that didn't work for you. Explain your issue with that particular part of the manuscript. But DON'T, I repeat, DON'T rewrite it. The author's job is to write. The editor's job is to edit.