When Ben and Wendy arrive at the diner, they have been on the road for at least a day and are somewhere in Kentucky. Nelson, meanwhile, had not been able to track them and at one point had made it as far as the Kansas City area (the opposite direction of Wendy) before realizing he had been thrown off. If Wendy called him from the diner, there is no way Nelson would have arrived just in time for Ben to realize Wendy had abandoned him.