I'm wishing the best for your DH. I think when most companies have a policy, it only has to do with one relative being in the "reporting structure" of the other. If they're in different departments then it would be no big deal or if they were both "worker bees" in the same department, it would not be a problem.
I would think since your DH and your DN would be "equals" then it wouldn't be a problem. However, it could present issues in the future if one of them wanted to be in a management position. (Maybe DN is worried about the impact on his potential career track?)
If they specifically asked your DH in the interview if he had any relatives at the company and he gave an incorrect answer (honestly, but mistakenly) then I think he should call first thing on Monday and set the record straight. If he was not asked directly in the interview, but it is mentioned later, I think he should just be honest -- that he did not know nephew was employed there until the nephew contacted him after the interview.
If it's a strict company policy (written or unwritten), then it is totally out of your DH's control. And he was honest in the interview, so he's done the best that he could. Sending wishes for good news though!