@TomD8 , I agree with your comments. In particular, he should file as a full year NJ resident.
It would be "rare" that would actually be a NY resident for those 4 months. But the fact that he notified his employer that he had changed residency and his employed withheld NYC income tax, may make it difficult for him to get a refund on NYC withholding.
His NY state tax will be the same (I think) whether he files as a non resident or part year resident. Whether he is subject to NYC tax is in question.
