Normally I would re-do the copy edit in one pass with a clean diff and cover additional points on their talk page regarding stuff that was not obvious from the diff. I would always open with comments on what they did right and thank them for participating. I would not copy edit as many articles myself during drives where I did a lot of reviewing, as it's time consuming, as there's teaching involved. Here's two sample reviews: User talk:Diannaa/Archive 6#Check yet another newb's work?; User talk:Diannaa/Archive 6#Review of copy-edit.