I was once a chronic, persistent nail biter. My mum introduced all sorts of remedies and deterrents over the years.
Much to her chagrin, nothing abated it. I remember horrible, blazing rows. In the end, she gave up. She eventually accepted that nail biting was just part of who I was.
I ended up quitting but it wasn’t until my twenties, am afraid. My nails and hands look just fine now and I turned out ok in life.
I wonder if there’s a way to frame it as a choice for your daughter. Perhaps start out by going to the place of giving her information. Like, 'Oh, I see you biting your nails. You know why it’s not good to bite our nails? Because there are germs that live under our nails.”
A way that frames it, in other words, as a choice. One that's hers to make. Leave the ball in her court and see what happens. She will grow out of it at some point.