Hi ethantd1,
When you manage her Apple Watch from your Watch app on your iPhone, do you see the Music app is installed on her iPhone?
It looks like you have already visited the article at the top of the page. To help isolate the behavior, take these steps to manage her Apple Watch from your iPhone:
Manage a family member's Apple Watch after you set it up
You can use your iPhone to adjust settings and customize features on a Family Setup watch:
1. Open the Apple Watch app on the iPhone that you used to set up the watch.
2. Tap All Watches, tap the watch under Family Watches, then tap Done.
3. Choose a setting to adjust.

Set up Apple Watch for a family member
Try having her ask Siri to open Music or play a song:
Listen to music and podcasts on your Apple Watch
If the issue continues, then reach out to Apple Support directly to help investigate the behavior. You can use the following link to reach out: Contact - Official Apple Support
Kind regards.