Many people complain about a sore or stiff neck while working out.
First and foremost your client should see a specialist to rule out any
serious problems which could be worsened by certain types of exercise.
If they do not have any serious problems they may have forward head
posture which puts a strain of the muscles of the back of the neck because
they are constantly contracting to try to hold her head back.
If she has forward head posture she should start a routine of neck
stretches and try to improve her posture.
Another common issue is tight upper trapezius muscles. Some people
have the tendency to constantly elevate their shoulders during periods of
stress (such as weight training).
You client should be made aware of this and try to avoid shrugging her
shoulders whenever possible. This would call for you to avoid any overhead exercises in which
naturally cause someone to shrug their shoulders.
Something you can do with exercise prescription is have her perform as
many exercises as possible from a supine position such as the stability ball