Left turn after touching first base

[fredhjr]

NFHS--I am BU. Nobody on, no outs. B1 hits ball to F5, who throws the ball into the dirt at 1st. F3 is off the bag and fields the throw as B1 crosses 1st safely, turns left and starts back toward 1st. Defensive coach is yelling at F3 to tag the runner, which she dutifully does. Coach yells at me She's out! I respectfully respond that she isn't. Coach asks for time and tells me because the runner is out because she turned left after crossing 1st safely. I explain to him that she made no attempt to go to 2nd (None. No feint, jerk of the head, juke, nothing.) After explaining this for the second time to the coach, I told him B1 remain on 1st. His parting comment--"You got that one wrong!" I have tried to find this situation in the rule book with no luck. The "Look-Back Rule" section does cover similar situations but the Look Back was not in play here. Comments?