Is it unethical for someone to proofread/edit your cover letter or CV?
I see no ethical problems with this whatsoever.  

One has to understand most academic ethical principles through the lens of academic society.  In particular the belief that academic writing should be solely that of the author unless otherwise scrupulously documented is not an absolute ethical principle but a belief and expectation of practicing academics throughout the world.  It applies to certain kinds of writing and not others (and there are edge cases that cause disagreement, as seen on this site).  

I don't know any academics who view a CV as a piece of academic writing in the sense that issues of originality and plagiarism apply.  If you like the format of someone else's CV, you can copy it for yourself.  In fact, it is more efficient if you just ask them for the word / latex / whatever file they used to make their CV and adapt it directly.  I imagine most academic CVs are adapted in this way.

Cover letters are viewed differently by different parts of academia.  In my circles (mathematics, research university, US) cover letters are usually quite 
perfunctory: within recent memory my colleagues and I discussed whether we even wanted to make a cover letter an official part of the job application.  In my field there is something called an "AMS Cover Sheet," which is a sort of form that you fill out that has the information of a cover letter, and some departments would happily accept this in lieu of a conventional cover letter.  Using an AMS Cover Sheet as a cover letter is the equivalent of adapting someone else's CV.  However, it is my understanding that in some other parts of US academia -- especially in liberal arts colleges and/or the humanities -- the cover letter is extremely important (I have heard someone say it is the most important part of the application).  If what you say to convince a hiring committee that you are a good fit for their institution is lifted from someone else's cover letter, then indeed that might be viewed as problematic.  

The above covers the situation of literally cutting and pasting CVs and cover letters.  When it comes to proofreading: it is my understanding and view that unless specifically prohibited, getting your academic documents proofread is not only acceptable but usually highly encouraged.  When it comes to editing, some amount is probably okay but some amount is probably too much.  It also depends on who is doing the editing.  My PhD advisor gave me more extensive feedback on my employment materials than on any other single thing.  I took the implication that these materials are extremely important, and I have tried to "pay forward" his help by giving similar extensive help to students (including, but not limited to, my thesis advisees) on their employment materials, webpages and so forth.  I would not hesitate to circle a sentence and scrawl a suggested alternative in the margins of their draft.  I do hesitate (and never have, I believe) retyped any of my students' theses or employment materials: thus I am suggesting changes -- sometimes very specific changes, and sometimes with the clear intent that my suggestion will be implemented directly -- but I am not making the changes myself.  

In general I have a hard time envisioning a proofreading or editing of a cover letter or CV that would be ethically problematic.  I think students and young academics should be much more concerned with having these and other application materials flawlessly literate and highly polished and should seek to err on the side of getting the help that they need to do so.  If such a person has a reason to think that some particular aid they are getting may cross the line, they should ask their advisor.