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Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
There are a few steps to this,
yes it does need to be on your home wifi network (internet connection required for transferring content).
However beyond that you'll have to deregister the kindle from her account (look in the account, about, or help section of the menu on her kindle) and then re-register it to your amazon account. Once that is done, your prior purchases should automatically sync to this kindle. If you have other documents of your own, you can transfer them via usb.
Let me know if you need more details on this process.