Something is wrong with Privacy settings, and the game has no rights to see friends from your friend list. To solve this use the recommendations below:
- Open your games list: https://www.facebook.com/games/manage
- Delete our game from the list. All your achievements will be saved.
Start the game again and give the game permission to see your friends.https://www.facebook.com/games/royal_solitaire/?fbs=106
If this does not help, your friend should do the same.