Today I saw multiple edits by the same user that seemed to destroy the formatting of the original question. See here and here. Other edits by this user do seem to be in good faith, see here. In both of the first two cases the edits were approved, and I rolled back the edit. Should I not have done that? What should one do if an edit is approved by the community although you're fairly certain the edit detracts from readability?
There are two separate issues here. Informally, what to do about the post, and what to do about the user.
If you are convinced that the current version of a post is significantly worse than a version you are willing to create, then you should not hesitate to create this better version. (Whether it undoes another edit or not.)
If you see a user constantly creating content that is problematic without them being aware, you should let them know in a friendly to neutral way. Note that you can comment-notify editors of posts (even though auto-complete does not propose them).
In case the above does not have the desired effect (for example the editor rollsback or there is some discussion starting), I would flag for moderators with "other" and describe the situation and move on.