I would like suggest that you can give friend a limit that he can help your island, while you are in vacations or something like that.
For example, buildings wells, fields, send explorer to treasure hunt or building mines.
You can choose for list, what friends can do and what they can't do, and you can choose friends that they have acces to watch your island.
Forbitten is :
Trades. So friend can't take extra resource from friend island.
Same system is in game travian.