Runner's Interference on Dropped Pop-Up

With no outs, the runner at first base (R1) is stealing on the pitch. The batter (B1) hits a fair high pop up into right field which should be easily caught by the right fielder. As R1 retreats back towards first base, R1 runs into the first baseman (who is standing directly in the baseline) and R1 falls down. 
 
Now the complicated part: As R1 is regaining his feet after his collision with the first baseman, the right fielder drops the pop up AND B1 rounds first base and briefly passes R1. The right fielder throws to second base to easily force out R1. The defense would like a second out called for B1 passing R1.
 
What do the umpires do now?
 
Call
Plays that have multiple rule issues are often the toughest to adjudicate. A runner is out when he passes an unobstructed preceding runner (NF 8-4-2m).
 
In the play below, R1 was obstructed, so in NF he would be awarded a minimum
of one base (NF 8-3-2). Since R1 was obstructed, there is no penalty for B1 passing him.