There are many institutions that require that the copy-editing only be made by professionals who have a degree in Philology, or an equivalent specialty (names can vary depending on each university). It is best for the copy editor to have academic training and proven experience in the area of editing.
The level of intervention
The copy editor draws attention on problems related to his area of expertise, that is, on the clarity and correctness of the way ideas are expressed, respecting and maintaining the original intention of the author of the text. The copy editor is expected to deliver a revision that will contribute to correcting and improving the text without completely rewriting or significantly altering the contents of the work. If he detects serious problems that would require considerable intervention on the material, he needs to report the issue immediately to the main editor, so that the latter, together with the author of the text, will rewrite the contents following the observations of the copy editor.
According to copyright laws, only form is protected, not ideas. Therefore, the editor has to learn not to pass that fine line and avoid incurring a violation of the moral right of the author over his work. At the same time, this mustn’t be an excuse for the writer to reject legitimate and necessary corrections from the point of view of the language or editorial standards that he has to respect.