assuming he is using a popular e-mail provider like Google, it should be relatively easy to re-gain access to it if it is associated to a cell number
... For that reason I've disabled all "restore options" a long time ago. If I lose access, it's gone.
Yes, thats is true SIM swap attacks are a thing and they are the reason one should not use one's phone number for anything money-related or perhaps in any case at all.
I simply find a bit baffling the fact OP did not have a backup plan to acceess his account, I mean, it does not have to be a phone number.
Security key, Google Autheticator, Authy, even a secondary email adress...
Heck, there are even emergency one-time use backup codes in for cases like this one, even thought the security of these are debatable, they can safe one's neck in situations like OP's.
https://support.google.com/accounts/answer/1187538?hl=en (Source)
Anyways, hopefully OP can find a way to recover his account.
I must feel terrible to lose one's account like this after the effort invested...