Thanks for reaching out, Chaoswrangler.
It sounds like your partner is currently part of an Apple Music family sharing plan, but they are planning on leaving and starting her own individual plan, or joining your family share plan. We can help.
There should be no problem with keeping playlists when leaving a family share group, but keep in mind that once removed from the family share plan, all offline songs will be erased because there won't be an active Apple Music subscription. That said, once a new subscription is created or joined, your partner can download their playlists and that should bring the music back.
It is important to know that playlist and song data doesn't last forever, so if the switch isn't made in a timely fashion that content will eventually be erased.
Also, there are limits on how many family share groups you can join. Learn more here: Leave Family Sharing.
Hope that helps.
Take care.