Unusual dropped third strike

[justmom]

This was the weirdest one I'd seen in quite a while. Two outs, no base runners, count is one ball/two strikes. Batter check-swings at next pitch. PU rules pitch a ball. Defense appeals to FU who rules it a swing, strike three. Defense heads toward dugout, batter walks maybe a third of the way back to her dugout when offensive coach yells at batter to run to first, because the ruled-on-appeal third strike was dropped. Runner runs to first (weaving through the mob of defensive leaving the field). Runner reaches first base while the defense was still trying to figure out what was happening. Defense was protesting loudly that "she can't do that". Runner is ruled safe at first on the dropped third strike. Should she have been out for walking away? (On the other hand, the defense should have known they were appealing for a third strike on a dropped pitch). So...how far do you let the batter walk away before you call them out?