Likely he is fine (besides being an unsafe idiot for drinking and driving).
If by “He received a DUI” you mean he was pulled over on that day and charged then he hasn’t yet been convicted, even by the definition in 46 crf. And at the time he applied he did not have a conviction.
If he holds a current valid credential then he has no obligation report a conviction until his next application of any type.
There is a lot of good information and applicable links and definitions here:
Ahh, well that’s a unique grey area. But my unofficial non-lawyer opinion is that he is still probably ok.
He didn’t have a conviction at the time he submitted the application and therefore he didn’t lie on his application. Based on the timeline you gave he also didn’t have a conviction when his application was accepted by the NMC.
He should probably count himself very lucky. And if he waits three years to apply for upgrade or renews in five years, and he only has the one conviction, then I don’t believe he’ll even have an assessment period…see Table 1 in 46 CFR 10.213 as referenced in the thread I linked to.