Eventually managed to sort it on my lads phone. They have some very persistent cookies it would appear on android. Logged out of the account, deleted all cookies in Chrome on his phone. Tried logging back in, using supporter number and password, still didn't work. So thought I would have one last go and clear the cache on the phone by booting into recovery mode.
Boom could log in and download the ticket to his Google pay. Wouldn't mind if it was an old phone but it's a 3 month old Google Pixel 5.
Strange that he hasn't had a similar problem with the other passes/loyalty cards he has on Google Pay so must be something odd with how Man City handle cookies or authentication.