How to recover the Zooba account in Game Centre

My son has two Zooba account in Game Center with different player/user names. We can see the account he wants to recover in his "friend" list but I don't know how to get to it and I don't know which Apple ID it was connected to and can't figure out how to access/recover the account. This is a zooba account he has played on for 3 years and we have bought multiple ad-ons for him. HELP!

[Re-Titled by Moderator]