Frog hurt by snake?????????????????????????

Me and my friend were watching the frogs in my pond when William (a small brown green frog) was ambushed by a small water snake. We vigorously hit the rock the snake was under and the snake let go within 15 min. when the frog jumped out of the whole we noticed he was breathing heavy from shock and his back leg was bleeding where the bottom portion met the top at the joint. What should we do? If we dont help we fear the snake will kill william.