If an employee is not happy with their performance review, they can raise a concern under the Company grievance policy.
By doing this a nominated company representative who is fair and impartial will investigate the performance review, meet with both parties (supervisor & employee) and give recommendation.
The employee needs to follow the company grievance procedure as quickly as possible.
The main thing for an employer to do is to remain calm and not make any comment or judgement on the situation. Refer to your grievance procedure, or alternatively contact SimplifyHR and we support and guide you with best practice while at all times ensuring you follow the principles of natural justice.