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It sounds like he turned on passcode lock on the Kindle. If he or you don't remember the passcode to login to your Kindle, you have to reset your Kindle to factory settings. Unfortunately that is your only way to access your Kindle.
Resetting will erase everything from it, but you can re-register it with the same Amazon account you used on it to redownload your books and other purchases without paying again.
Enter the incorrect password ***** multiple times and it will ask you to reset it. So do the reset.
After resetting the kindle, you can redownload all your books or stuffs to it using the same account you used to purchase them.
Please let me know if that didn't work, so we can try different method to reset it.
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.