Excel doesn't change CSV files when it imports them. If the imported file was being changed then the user was saving back to the same file they imported from.
The fact is the person was double-clicking a file in a list to view its contents and Excel was trampling it. Nobody in their right mind will waste time to open Excel first, use import feature, re-navigate to the file they were already looking at, and go through the import dialog just to see what's inside.