Hi rgirl-,
Thank you for participating in the Apple Support Communities. It looks like you came from the following support article: Use Messages on your Mac - Apple Support
If you are having issues sending messages to a specific contact, here are a few steps to try to see if they help:
1. Restart your device to see if the issue is still there: Shut down or restart your Mac - Apple Support
2. Confirm that your device is using the most recent software to rule out the older one: Update macOS on Mac - Apple Support
3. You mentioned that the message is being sent as an iMessage, did she use Apple products in the past? If so, let's be sure her number has been deregistered from iMessage using: Deregister iMessage on your iPhone or online - Apple Support
4. Try deleting each others contacts and adding them again to see if that helps.
5. Contact Apple Support here if you need further support: Contact - Official Apple Support
Have a great day.