You could make your own hair gel that is edible - 1/2 to 1 teaspoon unflavored gelatin and 1 cup warm water, may want to add something like vinegar so it doen't taste nice... otherwise she will end up licking it

ETA: Has your friend asked her daughter why she does it? It may give her a bit of insight and therefore a cure, in saying that though any habit is hard to break.

Deanne.