Illegal Slide on Force Play?

 
With no outs and runners on first base (R1) and third base (R3), the batter (B1) grounds to the second baseman.
 
The second baseman throws to the shortstop who is covering second base. After tagging second base to force out R1, the shortstop steps to the outfield side of the baseline to throw to first base in an effort to complete the double play. In turn, R1 slides toward the shortstop, about 3 feet outside a direct line (but still within reach) of second base. The shortstop jumps, and while there is no contact between R1 and the shortstop, the shortstop's throw is not in time to retire B1 at first base. R3 scores on the play.
 
Any actions needed by the umpire?
 
Call
On a force play, a slide is illegal if it is not on the ground and in a direct line between the two bases (NF 2-32-2f). The penalty for an illegal slide on a force play is that the runner is out, interference is called, the ball is immediately dead, and (with less than two outs) both the runner and batter-runner are declared out (NF 8-4-2PEN).
 
In the play below, R1's slide was illegal. R1 is out, B1 is out, and R3 is returned to third base (NF Casebook 8.4.2P).