Your BIL will have a fantastic voyage. Yes, check-in will require extra time, patience, a security photo and a medallion will be waiting (to be exchanged for a cruise card if he desires) but he will not be made to feel uncomfortable by any comments or actions from staff or fellow passengers if he chooses not to use the app. No one will argue with him to his face and he will be respected onboard - we are his cruising family. People only say non-complimentary statements about him, behind his back, here.