I assumed (perhaps wrongly) that Stacy intended to run until she ran into the other runners (who would have been on their way back) and then return with them so she would be running with the Cool Girls. But when I thought more, I realized that if they ran 5 miles, and the run was only 10, then Kelly/Stacy would have been ready to turn back or about to turn back already, correct? So the other girls would have passed them already on the way back to the car since they had been running faster.
As for the cell, when I go to the trail, I actually often leave my cell in the car because it can be a pain to keep track of it when I'm out there, so that doesn't bug me so much.