How to restore purchases in an app that is no longer available on Google Play

A good friend of mine played a game called "Cordy" years back. The first three levels in Cordy could be played for free, the ones after that you had to unlock via In-App purchase. Back then, she decided to buy premium. Fast forward to today: Her childhood game is no longer available on the Play Store, and when you install the APK provided by a third party, it comes without the stuff unlocked. Attempting to restore previous purchases doesn't do anything. Is there anything I can do to get it to work so that she can play the full game again? If I have to mess around with the APK file, that would be fine too.