30 days late means exactly that; 30 days late. It's a threshold, not a range. If your payment was due on the 1st, the bank can start charging interest and fees on that day. But if you pay the bill on the 29th, there will be no ding to your credit. So, 30 days late basically means you missed two payments.
If the bank actually reported it as 30 days late when it was only 1 day late, then a dispute should clear it. But that doesn't normally happen.
You sure? I'm not a credit expert, but looking at my credit report, there are only 3 entries for late payments, and it's 30 days, 60 days and 90 days. I don't know if they're not allowed to report "less than 30 days" or what they do for between 30-60, but from what I've heard a late payment is a late payment. He was 1 day late. They have every right to report him.
Even a 1 day late payment CAN get reported [doesn't have to, though, and most places don't report it, but they have the option to]: Even barely late payments can impact your credit score