How can I find that a friend of facebook accepted the invitation to play on iOS?

Question

I have an iOS game. And I want to reward the player after his friend on Facebook accepted his invitation to play the game. Now I see only one way to do this. Firstly, we save recipient fb identifiers, and then check from time to time that the recipients are connected to Facebook from the game (we check the is_app_user flag), and if so, we reward our sender. But in this case, we can’t determine which of the recipients started playing by accepting a friend’s request or found the game.

Perhaps someone has experience in this area and you can share a better strategy, how to track users who accepted a friend’s request to play the game?