As a book editor I deplore the bad workmanship of some of my colleagues. Editing a book takes time and care and the last step, for me, is running spellcheck to pick up any typos I might have missed. Quite often I have to refer a ‘correction’ back to the author to make sure I have understood the intended meaning. In some manuscripts I have to rewrite a sentence or paragraph that appears clumsy or obscure, and that may need discussion as well. An editor should always improve the material she’s working on and should have the professional integrity to turn down something that is really badly written. I agree that (your reader) should not pay the second installment, but tell the editor exactly why and cite examples.
Another thing, ask the editor you intend to use what she has worked on recently and who for. A good editor will be praised by her clients, something you often see in acknowledgements in a published book. And that may be the place to find an editor. If you aren’t writing fiction, look in books on similar subjects. Many editors who work for publishers do freelance work as well.