If the employee raises a grievance during the disciplinary process the employer can deal with the issue as follows:
If the grievance is totally unrelated to the disciplinary allegations:
It would normally be safe to progress with the disciplinary matter and deal with the grievance at a later stage.
If the grieveance essentially constitutes the employee's defense to the disciplinary issues:
It would be desirable to deal with the two things at the same time. For example a proposed dismissal for poor performance where the grievance alleges this was due to a manager's bullying. No discussion of the one could sensibly be carried out without a rehearsal of the other.
If the grievance seeks to criticise or cast doubt on the integrity of the individual who is to make the disciplinary or dismissal decision:
The safest course of action may be to adjourn the disciplinary hearing until the grievance has been resolved or to sidestep the grievance by shifting the making of the proposed disciplinary decision to another manager if the employer's hierarchy gives space to do so.