And being told they offended someone hits
hard. They never got the memo that the question would be inappropriate, especially that given their own accent, they have been asked the exact same question.
Why is there a rule all of a sudden? Why did I not get the memo? Why was it okay for others to ask me but not me someone else? Is there a special exception for this person? Is this a national culture thing? Can I fix this? I didn't mean to offend but I did!
That's what would go on in my head. I don't think that's unreasonable. If you want people to conform to your social or cultural norms, you need to explain them first.
And as is mentioned in the comments, assume good faith first. Educate instead of judge. Live and let live. Was there harm done by the question?