Why you should not have your friend edit your paper
Amateurs inevitably confuse editing with proofreading. Proofreading is a relatively small and less important aspect of editing. Proofreading is mechanical, and only concerns spelling, grammar, and other rule-governed aspects of writing. The difference can be compared with that between merely speaking a language at all and speaking it well.
Good editors also look for ways of rewording things that will improve the quality of the writing. And we look at things like organization, flow, and logic. And argument (making a good one, that will persuade readers). This is the principle task of every scholarly paper, which is addressed elsewhere on this page.
Good editing, like good writing, is "beyond good and evil,” to use Nietzsche’s phrase, when it comes to judgment of what is a good word, phrase, sentence, paragraph, or essay. Only at the most elementary levels is it a question of right or wrong. So editors, unlike proofreaders, don’t just catch errors. We make good writing better.
Also, your friend is probably not a professional editor of scholarly papers. Maybe he or she is a graduate student, even in your field. Not good enough.
We look at every word. We can do this and edit quickly because we have the experience. Some of us have been editing papers from students, graduate student, and professors full-time for a decade or more. Somewhere on a bulletin board at your school someone has advertised that they do this, and they are a graduate student doing this a few hours a week for extra cash. They might be good readers who will have good feedback, especially if they are in your or a related field; otherwise, they are amateurs. Skilled amateurs do things well, but not like professionals.