Gmail account appears to be fine, but the Amazon account has definitely been hijacked. Looks like the attacker texted a link that the neighbor clicked on this morning, and that completed some sort of account ownership transfer. Neighbor assures me they just clicked the link and didn't enter anything. They just landed on an Amazon page that said their account had been successfully transferred to someone else (they have a screenshot of the hijacker's email address).
They've been on the phone with Amazon trying to get it resolved, but if the description is correct it sure seems like there's a vulnerability on Amazon's end here.
At exactly the same time the SMS was sent the neighbor's Gmail account got hit with a firehose of thousands of spam messages persisting for several hours, which is why they thought the Gmail account was hacked (and also why they clicked the Amazon phishing link from the SMS).
Does this sort of thing sound familiar to anyone?
@jschuh The link sent via SMS could be the final 2FA verification click needed to approve an account transfer, with the rest having been setup beforehand. If so, the account itself might already have been compromised. (Always assume leaked re-used passwords)
caveat: I have not gone through an Amazon account transfer