So my brother in law opened an HSA because he was on a HDHP with work but later realized he was still on his parents health insurance so he didn't qualify.

He realized his mistake and called the HSA company who wrote him a check for his money back. But the money is still obviously considered non-taxable on his W-2. What should he do to properly file?

Not a CPA… report on IRS form 8889. He should also get a 1099-SA showing the corrective distribution.

Thanks that's basically what I told him but my concern is he got the money back in January so probably not properly reflected on the 1099-sa