If it was thrown backwards, it's a live ball.

If he throws the ball back away from the line, it is a fumble and live ball. If he throws it forward into the ground, it is an incomplete pass, and probably intentional grounding. 

The ruling is that the player fumbled twice.  The second fumble can be recovered by either team.

fumble, any incomplete backward throw aka lateral is a fumble

Intentional grounding..

(The ball would have to be thrown past the line of scrimmage for it not to be.)