There is a way to get it easily.
Have your friend drop 26 amount of Ceramic Coins to the crow in YOUR game (just in case, we let the crow finish his dialogue before dropping the coins) and (once the crow says "Sparkly, happy, happy!"). have your friend Save and Exit the game. Wait a few seconds then you must also Save and Exit Game. Once you come back, the red rusted key will be at the bottom of the tree.
This can also be used for duping the coins.
Maximum amount that can be duped at once is 25. Any amount over that will just result in the crow dropping the Rusted Key.
Note: Once you have the key, you cannot receive any more coins from the crow. So dupe the coins first if you want them, then do the key