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.
Your kindles are (or should be) each registered to separate amazon accounts.
You just need to sign out of your amazon account, and let her sign in to (or create a new account) hers, and then she can manage her payment setup for the kindle:
First, go here:
there you can be sure your card/bank info is correct (right address, valid expiration date, etc)
From there, go here:
On this page, click on the SETTINGS tab at the top right,
then you can from there set a default card to be used for one click purchases on the kindle.
Under Kindle Payment Settings, click on the Edit Payment Method button, and you can set the card you want used for your kindle purchases.