A copy editor is clearly of benefit to anyone submitting a PhD thesis. I beleive that it is indeed ethical to use such a facility. A copy editor I believe, will only correct structure and grammatical errors. I have read this in the book " How to get a PhD" which slags off people who dont know about this resource. Besides, nooone can ever find out that u used a copy editor anyways, hence from either viewpoint the argument of ethics is rather weak and irrelevant anyway.