Cannot use Apple Cash

1. Did you accept the money?
2. Is the money showing on your Apple Cash?
3. If you make your Apple Cash your default payment method in your Wallet app you can use it at a store - Set Up Apple Cash - Set up and use Apple Cash on iPhone (U.S. only) - Apple Support Use Apple Cash - Send and receive money with Apple Pay - Apple Support

You should get in touch with Apple support. I'm sure they can walk you through how to create an account. If you can't create one, the money should refund back to her Apple account but you might want to let them know what's going on. See if your friend can be with you or email/call them too. You might be able to get help from a local store but I'm pretty sure Apple support online is best place to start.

If a question is beyond your ability to answer, please skip it and go to the next question. A lot of people here on the forum can resolve this issue.