Job description advice

[Bivvysenior]

4 years ago a new Headmaster created a position of office manager with line authority over two part time office people (one being my wife).
The other partime clerk was dismissed. It left only my wife and the office manger. During the last four years the office manager had no interest or demonstrated any line management over my wife
My wife job description was never altered to reflect the new organisation and it clearly stated she reported to the Head. Last September my wife was asked to review her job description and was informed verbally by the head she will report only to her, the office manger also confirmed this. So the office manager was now left only managing herself. The reason for the job description review was to have everyone's job evaluated. This is about to happen now .The head and office manger now realise that under the review the role of the office manger would be downgraded. The Head is now trying to build up the Office manger job description with the reintroduction that my wife will report to the office manager and certain task will be removed from her to reduce workload in order to bulk up the tasks in the Office managers job description. As a consequence my wife's believes her job will be downgraded when assessed by HR.

I understand the right for the employer to change a job description
Under my wife's employers rules it says can only be done after consultation with the jobholder.

What is the best way forward as my wife has only a few years from a final salary pension and a loss of income now will have an adverse effect long term? It feels to me to be unfair for the Office manger to be protected at the cost of my wife loss of pay and long term reduced pension.

Any advice would be appreciated to take this forward.