Apple ID Verification Loop for iPhone Upgrade at Verizon

Hi all i work for a company that helps inside a large chain store sale the phones. I have a customer that activated his iPhone 10 years ago at Verizon. he says the rep created the id for him but he has not had access to that account since he upgraded his phone 5 years ago and he can not download apps or upgrade to a new phone because he keeps going in a loop with apple support about being able to verify the account. how can i help the customer get what he needs off the new phone and upgrade him a new one.

[Re-Titled by Moderator]