What I ... don't know how to do is edit my work. Not copy-edit, actually edit it; ... figure out what works and what doesn't and why. ... I have no idea where to even begin working on it.
She may not be the only one. So I'd like to publically offer a few immodest thoughts about where to begin working on it.
First of all, simply by worrying about editing your work, there is one fact you must realize: YOU ARE A BETTER WRITER THAN YOU KNOW.
Re-read that. Repeat it to yourself a few times (substituting the appropriate pronouns, unless you refer to yourself in the second person). There is a reason I wrote it in bold. And it is this:
Worrying about editing means that you CARE about your writing. You do not merely want to produce words, you want to produce CONTENT. You want to enhance your reader's experience. Considering the range of highly paid famous authors who can't be bothered to do this, simply by agonizing over editing you have already outclassed some professionals.
You are a better writer than you know.
But still. Editing is big and complex and scary. Where do you start?
The answer is: Write first, edit second. No, strike that: Write first, period. Let editing drift in organically.
Here are some good reasons why you shouldn't make a point of agonizing about your editing skill:
1) Paradoxically, simply by virtue of thinking you suck, you're better than most people. In between having the ability to assess your own writing skills, and the desire to improve, you're already doing enough right that anything you write in earnest will get you most of the way there.
2) Yes, you think you can't edit, but: Context.
2a) Consider dreck like the "Left Behind" series. This shows that popular acclaim, cultural influence and large amounts of money do not require ANY editing, not even slightly. Seriously, go read some of Slacktivist's ongoing takedown of the series that I linked above. You'll simultaneously feel much better about your own writing abilities and horrified that a major national bestseller could be done with such a total and complete lack of quality.
2b) Consider Isaac Asimov. He wrote 515 titles. Five hundred and fifteen titles. His writing career spanned 1939-1992, or 54 years. This is approximately nine NaNoWriMos per year, sustained, for his entire career. At that pace, it is basically impossible to do the sort of editing discussed above. Now, you're not Isaac Asimov, but you can still take heart in the fact that great, award-winning, wide-ranging writing does not always require editing -- and if you can improve your writing skill sufficiently, you might not need it either.
3) Reputable publishing houses -- should publishing be your intention -- employ professional editors whose entire job is to sit down with the author and suggest/enable these sorts of fixes. That won't help with the "my stuff sucks" feelings that are required by the author's job description, but it will give you an assisted chance to take a novel that's good and turn it into a novel that's great.
4) The editors in 3) above -- and your friends who could offer productive suggestions -- and writer's groups you may want to join -- can't help you edit unless you have source material to work on. Readers who offer useful feedback won't exist unless you give them something to read. In other words, how to "begin working on it" is to -- write more! :)
4a) Even if you never edit a given story, just having it written helps -- you learn what you like and dislike about it, and which of your experiments worked out well. Even if you never finish a given story, just having made the attempt helps -- you see where you stalled and learn what slows and stops you.
4b) The more you write, the better you get at identifying flaws in your writing style. "Hey, my last four stories have all stalled out on romantic dialogue. Now I know what I need to focus my editing mojo on!"
To round out the list at an even five:
5) The best editing that I have ever done has come about thus. I finished writing a story, or hit a stalling point. I let it sit untouched for long enough to have completely forgotten it. I came back later, and saw it with fresh eyes, as a reader. Then I reacted to it as if it were someone else's, criticized (or enjoyed) it accordingly, and took my own advice.
This is a lot easier to do if you're producing enough material to drive old or stuck stories out of your mind. If you keep thrashing at a single tale over and over and over again for months at a stretch, soon you completely lose sight of what simply reading it is like. In order to edit it productively, you have to stop editing, keep writing, and then pick it back up again later.
Besides -- I've discovered some of my own best writing this way. It's one of the greatest joys of a creator to dig back through your old corpus, exhume a few bodies, and then find out that they don't actually stink.
 Maybe I'm not the best person to talk about editing. After all, by the time I reach the end of a story I'm generally done with it. (On the other hand, I keep a mental sketch of the entire plot in my head from the beginning; I write in second drafts; and I immediately re-read everything I produce and microedit it again; so everything that comes out of my fingers is generally about as polished as it's going to get anyway.) On the other hand, I could argue that that strategy has gotten me this far, and so I've got to be at least as much an expert as anyone else who wishes to speak up on the matter.[^]
 This is NOT to say "ignore editing." By all means, get feedback -- experiment -- take advice -- read how-to- books. But these are not things you should worry about, because as you write, you will end up working on individual little pieces of the Giant Editing Puzzle, and by doing a lot of writing instead of a lot of editing you'll end up with a lot more output and just as much knowledge.[^]
 Also consider the "Criticism" section of his Wikipedia article. I think we can all agree, if only on the strength of his awards, that Asimov was a great writer. And yet there's plenty to hate about his writing. You will always have legitimate criticism to contend with, no matter how great you get. Past a certain point you have to say "fuck it" and just be a great writer anyway.[^]