I used to be one of those squidding passengers many years ago. I didn't understand the dangers and implications of refusing to wear gear back then, and I, like so many others, trusted the pilot implicitly and believed myself to be immortal. No one took the time to explain to me the damages that sliding down asphalt can cause. I was incredibly naive and thankfully nothing ever happened to me aside from a badly burned leg from an exhaust pipe once.
It's your job as the rider to explain this to your passenger and impress upon them that they're not immortal, that asphalt hurts, and that even a simple low speed fall
could kill them. It sounds to me like you're already doing this, so a big KUDOS! to you.
I don't personally take passengers, but if I did, I would refuse to take one that wasn't ATGATT. Perhaps you should show your passenger some vids and websites about this to scare the gear right onto her body.