particularly new clients often have overactive traps and have a really hard time disengaging them. This can look like the head is going forward and the neck hyper-extends. Those movements can be very subtle and still cause discomfort in the neck in the way your client is describing it.
I would try a triceps exercise where she presses down and watch for elevation and forward rotation of the shoulders. Often times, I have clients just work on strengthening the lower traps to keep the shoulders depressed before I have them do anything else.