Referencing an earlier topic I found, is the dev in question getting an error like this?
Confirm your payment method to keep using our platform. We think you are a human but our system does not. Click here to confirm your payment method to unlock your account.
In that case, it may want one as a safeguard against abuse. Although if all plans are paid now (closing the door to abuse of free tier) and they only have access to create resources through your org that does have a payment method attached, I’m not sure if that makes sense. Have you tried contacting billing@fly.io directly about getting your org members’ accounts unlocked?