One should care for the poor man. Of course, it's difficult to control a horse dragging something in its stirrup and they must be gently guided back and forth over the obstruction until it is removed. By then the para-medics should have arrived and I'm sure they will laud your attempts to control further dragging. As Kat said, "Keep your tongues in cheeks and your feet in the stirrups." Maybe the tongue/cheek thing isn't a good idea if one sits the trot.