Hi
Like production operator, Food Safety Team Leader, Supervisor, Engineer, Chemist, or anyone who plays a role in the system and or who has bearing in food safety or customer satisfaction must be competent. So, I think the requirements are determined by a certain program/standard, according to what performance is desired from an auditor.
Common minimum is an auditing course for the applicable standard. Other possible qualifications are education, knowledge, skills, a period of experience, and others as they might apply.