First Base Question

[LDUB]

I know I read a post about this no too long ago, but it might have been on the softball board.

So here is what happnes. R3 and 1 out, infield playing in on the grass. B1 hits one hop to F4 who throws to F2. R3 is called out at home. But the BR thinks that was the third out. BR runs through first base, then he turns and runs back towards the third base dugout. The dugout was pretty straight behind the foul line, so the BR ran back to the dugout in the baseline. The defense threw the ball to F3, who was on the base. Then he went and tagged the BR, who by then had realized his mistake and was running back towards first.

Now I know 1st base is not a force out, so if the BR retreats back past first, does the defense have to tag him out, or do they just have to throw the ball to F3 and have him tag the base? Does it make a difference if the BR steps on first on his way back to the dugout?