Like you said, it is wrong, so my suggestion would be to have it as a potential ‘right’ answer, but with the stipulation that it is wrong (in the orange text)
Also keep in mind that in higher level business situations, people get in trouble for misusing keigo quite often (well… Not in trouble, but left feeling embarassed). I had a client a few days ago that told me that he had to apologize to his subordinate, because he accused him of making a keigo error in his report, but when he showed a higher level boss, that guy said that the subordinate was actually correct. So in the end, my client tried to call out another persons error, and ended up getting schooled in keigo himself. Hahah.