For whatever reason when my son got his tablet a couple of years ago he didn’t set up his own Google / Play Store account, but logged on his sister’s instead.

Can he create his own account without deleting the history on the only game that he plays? He has invested about a billion hours on this and to start again would cause an epic meltdown.

He’s not bothered about anything else, just this one game. The problem has arisen because he downloaded other stuff on the account that his sister now wants, but can’t get.

Best solution would be for him to have his own so he can do what he wants without the need for a stand up row at 6:30 in the morning that probably woke up the entire village.

best answer would be “ask the game developers”
https://support.google.com/googleplay/thread/5053822/transfer-google-play-game-data-to-a-new-google-account?hl=en

He’s not bothered about anything else, just this one game. The problem has arisen because he downloaded other stuff on the account that his sister now wants, but can’t get.

Reading that my first thought is remove the other stuff, create and register his account on the device alongside his sister’s, reinstall the other stuff on his account on that device.

FWIW I have 4 accounts registered on this device. 90% of the apps are installed as main ID, the others – eg calendars – are installed on shared id.

All works fine.

best answer would be “ask the game developers”

This. Some games as well as saving data to Google Play also sync data with the game devs servers. An example would be AdVenture Capitalist, it uses the devs server time so you can’t use your phone’s date/time to cheat and get loads of progress for no work. Sometimes you can see this as you’ll have unique ID which shows up on loading screens or is hidden away within the depths of the settings menu in the game itself.

Another option is that sometimes a game will allow you to link progress to a social media account like Facebook. All depends on the game to be honest.