Delete game save from Apple Game Center

I was playing a game on my iOS device and wanted to show a friend so he would start playing too; so I logged into my Game Center on his device and loaded my city.  I then logged out and he logged to his account and deleted the game from his device so as to start the tutorial but after doing the tutorial he went to save his game to his Game Center profile but it wont let him, it says a city is already saved and if he loads the city that is saved it loads my city.

How do we delete my save from his profile?  It is strange because if he makes a change to the game on his device it makes the change on my device even though we are both have different Game Center (and Facebook) ID's.

Where is the info being saved that is syncing the game on our two distinct accounts/devices?  Major security concerns arising from this!