Hi David Goodall,
Thank you for using the Apple Support Communities! I understand you have a question about Apple Music with HomePod, and I am happy to help.
When you sign up for an Apple Music Family subscription, and set up your Family group, everyone in your group will automatically have access to Apple Music: Get an Apple Music family subscription - Apple Support.
When your partner sets up her new HomePod with her Apple ID, she will be able to AirPlay Apple Music to her HomePod following these steps: Play audio on HomePod using an iOS or iPadOS device - Apple Support. It would not matter if the Apple Music subscription was under your Apple ID (as the organizer), since Family Sharing is based on inviting users that have their own separate Apple ID's already.
Best regards.