I have a friend whose daughter used to bite finger nails a lot. My friend had tried many methods help her daughter to quit, but all failed. Finally, one day, my friend nail clipped for the daughter and put all the the nails on a tissue, as usual. When she'd finished, she told the daughter to eat all the nails, because that's what the daughter was biting everyday! The daughter looked at her and didn't say anything. Of course, she didn't eat but she never bite nails since then.
Perhaps you can try this method. It would be very disgusting. But this can tell your daughter she IS actually eating the disgusting stuff.
Hope it helps.