If you messaged her money using Apple Cash, there is likely nothing you can do. When you pay with cash, there are no purchase protections like you would get on a credit card. Never send cash to an unknown person. If you handed someone cash on the street, they are under no obligation to give it back to you.
Here is the Support Article for sending Apple Cash:
Send and receive money with Apple Cash - Apple Support
The relevant portion is:
Cancel or dispute a payment
- Open the Messages app, then open the conversation and tap the payment.
- Your Apple Cash card will open in the Wallet app. Under Latest Transactions, tap the payment, then tap it again.
- Tap Cancel Payment.
If you don't see Cancel Payment, the person has already accepted the payment. Try asking the recipient to send the money back to you instead.
Depending on your bank or card issuer, it might take 1-5 business days for the money to be refunded.
Apple Cash person-to-person payments are like any other private transaction between two people, so only send and receive money with people you know. If you have questions or concerns about a transaction, please contact us.