That actually sounds like a very reasonable mistake for someone to make. I'm not familiar with exactly how the dropbox UI works (if they would have gotten prom

That actually sounds like a very reasonable mistake for someone to make. I'm not familiar with exactly how the dropbox UI works (if they would have gotten prompts or warnings), but it does go outside of the typical users mental model if deleting a file on their drive means that the file gets deleted on the server and their friend's PC as well.