To see who among your friends need help with order on their Order Board, tap the Friend button on the left side of the game screen.
Next, tap on the Ask For Help tab. Then tap on the Visit Farm button.
If you are a member of a Co-Op, you may also see requests for help in the Co-Op chat, and by tapping this message you can visit your neighbor's farm to help them out with their Order Board.
When you're on a neighbor's farm, you will see a bubble with a purple Ask For Help icon on top of their Order Board. A green checkmark on the bubble indicates that you have the required items to help your neighbor out and fulfill their order.
After tapping the Order Board, you can select the Ask For Help order and fulfill it using your own items by tapping the Help button. You will receive the Coin value of the order and a Help bonus as your reward. By helping your neighbor, any of your ongoing tasks or quests that require this order will also progress.