TCC
I've never heard of such a thing. The problem with volleyball is that,
due to it being a team sport and most people not wearing headgear,
anything hard is porbably going to break the rules ("4.5.1 It is
forbidden to wear objects which may cause injury, or give an
artificial advantage to the player.").
However you may find that some of the headguards designed for rugby
may be of use so I would check them out - try googling for "rugby
headguards" and see if that helps.
She will look bloody silly though.
That's pretty interesting.
There are soft helmets that epileptics wear, and some are marketed for
snowboarding too.