How do you cancel a PENDING purchase made in the APP store by mistake?

You can request a refund from her:

https://reportaproblem.apple.com

If you can not choose a refund, because it is written as "Pending" - then wait until the purchase is completed, and then request a refund.

The purchase is not pending. Purchase has been completed in the App Store. What is pending is the payment. This pending payment will post (complete) in a few days and then you may request a refund here ➔ Refunds, Subscriptions & Billing Questions

Axel F.