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Re:Apple Is Blocking Linux User-Agent on appleid.apple.com by tinus_hn on Saturday February 23, 2019 @12:00PM (#19234171)
You can actually add more than one number so he could have added a family members or friends number. He might also be able to use the recovery procedure from this link to regain access after a waiting period:

https://support.apple.com/en-us/HT204921

Re:Apple Is Blocking Linux User-Agent on appleid.apple.com by throwaway45901 on Sunday February 24, 2019 @09:27AM (#19238657)
Yes, if he had known this would happen, he could have done all those things.

Or if Apple did not force 2FA on new accounts, then this also wouldn’t have happened.

Re:Apple Is Blocking Linux User-Agent on appleid.apple.com by tinus_hn on Monday February 25, 2019 @02:11AM (#19243656)
But they do and for very good reasons. Too bad your friend got caught in the cracks but it’s his own fault.