Finding a good, professional editor is not easy. My company gets applications weekly from people claiming to be professional editors, who do have some experience, but who cannot pass our editing test.
Before hiring an editor, your friend should understand the relationship between an author and editor. To that end, I suggest your friend read
Symbiosis: The Authorial and Editorial Process.
After that article, I suggest your friend read the following articles, and the articles that are cited (and linked) therein, that tackle various aspects of professional editing. The more your friend knows and understands about professional editing, the more likely your friend will find a professional editor who matches your friend's needs.
Good luck.