The NJ FFL will pin on a muzzle brake and cut off the bayonet lug to make it legal for him to transfer the rifle to the purchaser. Otherwise, the FFL is committing a crime if he transfers it as-is. Either way, its not on you.
However, the FFL, if he's not aware of these particulars, and isn't equipped to perform the necessary modifications, or is unwilling to do so, may tell the purchaser that he can't legally transfer the rifle, and then the hapless purchaser may come back to you looking to ship the rifle back and ask for a refund. IMO, you are under no obligation to honor that, as its not your fault the purchaser didn't understand the laws of his state before purchasing.
ETA: Since it sounds like this hasn't shipped yet, I would just confirm with the purchaser that he understands that the rifle will need to be modified to be legal, and ensure he has informed his receiving FFL of this.