Recover email without an alternate email or phone #?

My father’s email may have been hacked. He got a notification asking if he changed his password, which he didn’t. When he tried logging in, it prompted his password which either was changed or he didn’t remember the old one.

Either way, trying to recover it, a new link was sent to his old email, which he forgot the password too. He did not link his phone # to the email, so that’s not another way to recover it. Tips?