Editor's fees?

I would pay her based on what her credentials are, and balance it with what the going market rate is (can't help you with that, though, sorry). Maybe someone around here can pitch in with some links to the info. I know you can find it online somewhere. If I had to guess, though, I would wager at least 20$/hr at the *very* least. Either that or you could offer a flat fee based on how thorough of an edit she does.

Whatever you decide, please make sure that she is qualified. It's great that she is willing to do it, but being an avid reader does not necessarily a good editor make. I'm an avid reader, but no way would I take anyone's money to edit their work because I don't have that background. I worry that although she may do a decent job, if it's not up to par with what a professional editor does, and you get no responses on your query, it could potentially lead to resentful feelings. I would just suggest weighing the pros and cons (cost, convenience, etc.) of using this gal vs. an established, reputable book doctor. Good luck!

Clara is right on. Avid readers do not in any way translate automatically to capable editors; many editors are avid readers, but make sure you check her credentials and her experience before you negotiate anything. Will this editor be doing a content/developmental edit, or a copyedit, or simply a proofread? All are usually charged at different rates. Google the going rate for freelance editing and you'll have some idea of what professional freelancers are charging for their work.