The ball came out of the far side of the tunnel before he touched it - reset scrum and throw in again (20.7(b)):
[LAWS]If the scrum half throws in the ball and it comes out at either end of the tunnel, the ball must be thrown in again unless a free kick or penalty has been awarded.[/LAWS]
Alternatively, if you don't stop play at that point, the flanker brings the ball back into the scrum, so it's a FK under Law 20.9(d):
[LAWS] All players: When the ball comes out, leave it out. When the ball has left the scrum, a player must not bring it back in to the scrum.
Sanction: Free Kick[/LAWS]
But in either event, it's not a PK under 20.9(b) because he doesn't handle the ball in the scrum.