What would you do if another NCO corrected your soldier and was wrong on the regulation?

Ask my soldier to wait outside and educate the board. Once the board is in agreement that my soldier's uniform was correct, I would go outside the board room and explain to my soldier that the board has checked the regulations and found that there was no discrepancy with her uniform. I would then have her report to the board to resume the interview. If the senior female board member has any integrity, she will explain her error. If she is unwilling to do so, I would expect the senior member of the board to reinforce there was no error in her uniform, tell her not to be concerned with the error and proceed with the interview.
If I were the CO, and the senior female NCO did not explain her error to set my soldier at ease, she and I would have a meeting on the mountain after the board was adjourned.

Similar thing happened to me. I had presented to the board with my sponsor in Dress A's. The board members were in ACUs. They looked over my uniform and told me I was wearing the wrong unit award and I needed to get my shit right before attending their board. They went on to tell me that they thought I was 'the example' for junior soldiers and that this was extremely disappointing.

After I left the board, I was searching my phone to find where I went wrong and made sure I had the right unit award. Shortly after I was kicked out, my sponsor pulled me back in to complete the board. At the end of the board, the members apologized, saying they were comparing my uniform to my sponsor's uniform and thought I had the wrong unit medal on it, when my sponsor was the perpetrator.

So, my sponsor stood up for me and I formed the board members that I was in the right, but he did it privately and everything worked out in the end.

I am distressed the rest of the board did not react or know the regulation. Shame on them.