Interference, Obstruction, or Nothing?

This was in a slow pitch softball game, but curious about what the ruling would be in baseball as well.

Situation: runner on first, batter hits hard ground ball back to pitcher. Pitcher quickly gets the ball and throws it to first base. R1 starts running toward second base when the ball is hit, but then incorrectly thinks he has to go back to first base for some reason. The throw is ever so slightly to the outfield side of first, and the first baseman has to position himself directly in the lane between first and second to catch the ball. The runner that is running back to first unintentionally runs into the first baseman and causes him to miss the ball.

What is the call: Interference on the runner, obstruction on the first baseman, or nothing?