Okay, something really dumb happened to my friend.
He submitted an Android application a few weeks ago that offers In-App Purchases to the user.
Now, he needed to update the .apk to fix a bug, but he actually lost the keystore that he used to sign the application the other day (what a dumbass...). He tells me it's definitely lost. He exhausted all options to find or restore it, but without success. So, as he understands, the only way to submit the update is to actually create a new application in the Google Play Developer Console. He is so dumb, I always tell him to do backups of important files, he just won't listen (wow, really, such a blockhead...)!!!! sigh...
Luckily, he only had a handful of users who actually purchased something in his app during that time. However, I would like to know (for my friend) if there is any way to transfer the In App Purchases from the same user from the old app to the new one, so that the users who bought something in the app and now want to update it, won't get punished for his stupidity and have to purchase the same items again....
AFAIK, there is no way to do that. The only option is to refund them and ask them to buy again in the new app. I believe refunding can be done by logging in to Merchant account.
Google Play does not provide any form of content delivery. You are responsible for delivering the digital content that you sell in your applications. In-app products are always explicitly associated with one and only one app. That is, one application cannot purchase an in-app product published for another app, even if they are from the same developer.
And BTW, Does the app have any backend? Where in he saved data of all purchases made?