Google customer service is absolute shit when you're trying to do anything that their automated stuff can't help you with. Is she signed into any other devices, like Chrome on a PC?
A quick search brings up the 5a was prone to motherboard failures, so that may be your issue.
Google even offered a 2 year extended repair program for it. If you can find a replacement motherboard for a reasonable price and are feeling adventurous, you could try
replacing it yourself. If all you need to do is get it functional enough to validate 2FA, you don't need to do a perfect job, just make sure not to completely break the display.
This kind of shit is why I hate 2FA that doesn't have redundant methods built in and am hesitant to use passkeys. I want to be able to log in to important accounts like Google with the the contents of my brain and not be dependent on any devices that I might not have on me or have failed.
Edit: Just noticed that you said you don't think she's logged into her account on any other devices. That sucks. I know that I had to recover my Dad's Google account maybe a decade or more ago and there once was an option to validate a bunch of info about the account and have them send a reset link to another email. Since then, I have my account set as the recovery account for his.
