Hi,
Long term joint manipulation is likely to cause ligamentous laxity at the facet joints that in turn results in perpetually tight cervical musculature because the brain interprets cervical laxity as dangerous to the nerve roots leading to a splinting of the muscles via increased resting tension. Ongoing muscle tension increase can give hyperplasic over reactive muscle, thgen the cycle repeats. Good for the over enthusiastic manip therapists that make lots of money from deluded but wrongly grateful patients!
Correct physio treatment requires patient re education and understanding of the problem. They will be keen to understand that money will be saved once the muscle / posture re-education is completed.

Try C6-7 facet joint mobilisation, but you will need to do quite deep (very uncomfortable) mobilisations to be effective. Prior to mthe mobilisation give deep friction massage to the Levatore scapulae origon onto the scapula then the same frictions over the lower scalene points on the facets (5,6,7). Tell the patient to do Levatore scapulae stretching exercise.

The reason for all the above is that the C6-7 level can refer to the inferior angle of the scapula, orbital facial headache and to the ipsilateral upper limb along the triceps and wrist extensors.

If the above does not assist quickly enough, try trigger point dry needling to the Levatore scap and scalenes.
Good luck and let me know how you get on.
Cheers,
MrPhysio+