I can't say for sure, but there's a good chance you/she have paid for the first month with a commitment to have it auto-renewed every month until explicitly canceled. If it came with a free trial, you/she should be able to cancel before the trial runs out to avoid any charges.
Log into your/her account at my.roku.com and check the status of any Roku-managed subscriptions, under My account.
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If you signed up through Roku Pay and need assistance with your account, please reach out to our support team directly here for your 'Account or billing' issue, and select 'managing your order': https://support.roku.com/contactus. They will be able further assist you.
However, if you signed up for a Pandora subscription directly through the channel provider, you will need to contact Pandora support to assist you further.