Every time you hit a chapter milestone - halfway and at the end - you get two rewards: one for yourself and one to send to a friend! These moments are clearly marked in your chapter progress, as shown in the image below:
When you reach a milestone, you’ll open your own gift first. Then, a list of your friends who play the game will pop up so you can send them a gift, too! If none of your friends play yet, you can invite someone new. Hopefully, you’ll have someone to share with next time.
If a friend sends you a gift (or you send one to them), you’ll see a notification in your Messenger conversation with the sender:
The friend gift always contains 1 Joker, 3 Undo, and 150 coins.