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If you create an account and sign in to that account on your iPad's Kindle app, you will lose all the books purchased/downloaded using your friend's account. If that is OK, then all you have to do is de-register the Kindle app and re-register it to your account or you can delete the Kindle app, redownload it from App Store and register it with your account.
To de-register Kindle app and re-register to your account, follow the steps below;
Then open it and it will ask for an Amazon account. Just register it with your Amazon account.
If you don't have an Amazon account, you can either register one from within the Kindle app or you can register one from here --> https://goo.gl/u3EOBo
Remember, this is not my final answer. If my solutions didn't work please don't rate me bad or poor. Instead please reply back to me and I will continue to help you until we fix the problem or you are satisfied.
You can also delete the Kindle app and redownload it from App Store to register it to your account. To delete, just hold down the Kindle app in the iPad home screen and it will start wiggling. Tap the x icon over it to delete it.
Then open App Store, search Kindle and install it. After installing Kindle app, open it and register it to your Amazon account.
If you have any difficulty on following the steps above, please let me know and I will walk you through it.