[AtlUmpSteve]

And my $.02 follows the thought process of the last few posts. The pitcher catching a line drive is a defensive player making a play; the fact that it is F1, and happens to be in the circle isn't an instantaneous transition to LBR status. Given that runners are 1) required to complete running responsibilities, and 2) allowed reaction time (subjective time based on age and level) to recognize that the pitcher has control of the ball in the circle, it is reasonable to allow any continuing play as a legitimate play.

To me the issue in the OP is how quickly the runner makes the transition. If the runner returns, tags, and immediately moves to 2nd, I have a continuing play, and don't believe that the reversal of direction (which is obviously required to advance) is a violation. If R1 returns, stops, looks, sees F1 ignoring her, and then takes off, I have a LBR violation. Obviously, as suggested in an other post, if F1 is making a play or faking a play on another runner, this is all a moot point (but giving the benefit of doubt to the original post, there can be no other runner in softball nomenclature if R1 is on first).