Batter-Runner Passing Baserunner

 
With no outs and a runner on first base (R1), the batter (B1) hits a high infield pop-up near second base. R1 fears that he will be doubled-up, so R1 retreats back to first base and is standing very near first base. Meanwhile, an aggressive-running B1 touches first base and curls into fair territory (hint, passing R1).
 
However, the second baseman unintentionally drops the pop-up, but recovers the ball and steps on second base to force R1. At the end of the play, R1 is standing on second base and B1 is standing on first base. The base umpire has a lot to do on this play. What are his calls?
 
Call
When a trailing runner passes a preceding runner, the trailing runner is immediately out (NF 8-4-2m). In many cases, this may remove the force play on the lead runners. In the situation below, B1 is called out the moment he passes R1. This removes the force on R1. In order to retire R1, the defense must now tag R1 before he arrives at 2nd base (or retreats to 1st base).