I agree with @Family: he will have to respond to the NOID. Given the circumstances, I'm surprised the officer didn't ask him about his marriage and taxes. 1) Though he stated in the application for naturalization that he was married, he most likely stated different addresses for him and his wife for the past 7 years. For some reason, the officer didn't ask him why they lived together only for the first three years and why they have been living separately afterwards for the past seven years. It may be a misrepresentation in their eyes (just a supposition). The good news here is that he is still on good terms with his estranged wife, who can vouch for their truthful marriage in a written notarized statement. He just needs to ask her to write it down, notarize it, and send it to him asap. I would also include any evidence of mutual co-living for the first three years.
2) As far as I know, they usually ask at the interview if you have ever owed any tax payments. This is one of their standard questions. If this question wasn't asked, that's one story: he can provide proof of his payment plan in his response to the NOID. If he was asked this question and he somehow misrepresented himself by withholding the truth or otherwise, that is a different story. If he has consistently owed taxes for a number of years in a row, it may also have added additional complications to his case. If I were in this position, I would do everything possible to pay off the whole debt today or tomorrow. But if he can't do that at all and/or if he owes taxes only for one year, then at least provide them not only with a freshly created payment plan, but also that he's been doing monthly payments for the debt period, whatever months/years it may be.