I would say that you would probably have to a value which saves on their device.
I think it would be done by adding the local storage in to your game
then add a action
Local Storage>Set Item
Key is the name of the file
Then Value I would do 1
1 = Shared
If the player has not shared it there will be no storage, so when checking when they press share, do check item exists.
I would show screen shots explaining it as I am a bad person to explain things, but I don't know how (Yes i do know print screen, but you can't just paste it in)