I agree. You can use Track Changes and set it to display changes the way you want, or you can mark the text you add. The simplest way to do that is to use a character style, add that style to the QuickStyles pane, and click on it before you add anything. Of course this does not deal with deletions. You could have a character style to apply to things you want to delete as well, I suppose.

Perhaps, it may be time for your correspondent to learn to at least view tracked changes?