As for "what now", you're right again... it's their issue, not yours. You seem smart and respectful of yourself and of them, so I'm sure you'll do a good job of figuring out how to keep a little distance if necessary without losing the friendships. And hey, of course you can't be angling for this in good conscience, but... many people just learning about poly need time to adjust to the idea. No guarantees, but I'd say theres every possibility that she'll come around in her own time.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.