Who petrified Greyback?

[festuco]

No: HPFGUIDX 145353

When Harry follows the DE's from the tower he is attacked by Greyback,
who almost kills him when he suddenly is petrified. I've been trying
to figure out who did this, and I have no clue. When Harry gets up and
starts running after Snape we get an overview from the battle and
everybody is duelling. Except two bodies. One dead DE and Bill. But
Bill was heavily wounded. So who saved Harry? 

Gerry