How to approve my old iPhone for boyfriend

My boyfriend fell off a boat and ruined his iPhone SE (1st gen) I have an old iPhone 6 that I erased and completely reset that he can put his SIM card into and he’s setting it up and signed into his Apple ID but it’s asking to approve this iPhone? Saying go to one of your other devices to approve this iPhone. But not sure how to do that when the only other device he has won’t turn on because of water damage, can I use my own iPhone SE (2nd gen) to approve it? How do I do this lol help please & thanks

[Re-Titled by Moderator]