That would require us to manually remake this user account

That sounds fine? Just add it to the script when down-syncing. Or keep auth details in a separate DB and only sync that as needed (that’s what we do).

The customer is paranoid, as the project is their public facing website, so they want testing against the actual prod environment.

That’s the main problem then, not this testing engineer. We do test directly on prod, but it’s not our QA engineers doing the testing, but our support staff and product owners (i.e. people who already have prod access). They verify that the new functionality works as expected and do a quick smoke test to make sure critical flows aren’t totally busted. This covers the “paranoid customer” issue while also keeping engineers away from prod.

Maybe you’re doing something like that now, idk, but I highly recommend that flow.