If this was done properly, the employer would not have reported any wages to NJ or withheld NJ taxes.

As you may already know, NJ wages are different because NJ does not allow pre-tax deduction for retirement plan contributions. But that has nothing to so with how the returns should be filed.

Are you saying the employer had withheld both PA and NJ taxes at the same time and your client never complained?  If your client never turned in the NJ-165, the employer will not issue a W-2C; that's to be expected.  But if taxes were withheld to both states, there's something fundamentally wrong with the employer's payroll process.

As for the mechanics and technicalities of how you prepare the NJ return, please see my suggestion in the earlier post.