Go to https://my.roku.com and log in using the email address that's displayed on her device under Settings->System->About. You can add a PIN so that no purchases can be made without it, or, better yet, remove the payment method associated with the account. You can only do that if you have no current subscriptions through Roku.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.
