I don't want to restart the debate, but we recently had a heated discussion about whether there shouldn't be an option to have "None of the above" as the last choice in multiple choice questions. Although this choice is found in many official tests, the suggestion to include this option was rejected out of hand.
So for your suggestion to include "I don't know" as the last choice, well, good luck.
To troubleshoot your colleague, you could suggest that he include a small script in his questions. It's almost as easy as having an option in the question's settings. This is of course a temporary workaround until your suggestion is adopted and implemented.
Here is the css. If you include it in a <style> tag in the question text, don't use the Atto editor, use the Plain text area editor otherwise it will be deleted: