The second the interviewer changed the parameters people would start complaining that they were "Setup to fail." These threads commonly illuminate that interviewers simply cannot win, regardless of how genuine they are.

That's why I recommend confirming that the candidate's first solution was right. Use lots of positive feedback: ok, looks good, that's right, that's how I would do it; I see where you're going with this; etc. Then introduce the change, with a justification e.g. the team providing the input data changed their format/guarantees/technology, and we have to blah blah blah.