If there is a dispute then the employer must use the disciplinary process. The statutory minimum requires the employer to:
Inform the employee of the problem
Hold a meeting to discuss the problem
Decide on outcome and action
See the ACAS website for details of
Discipline and Dismissal Procedure. Or call the ACAS Helpline 08457 47 47 47 (M-F)
In your case I'd imagine that the volunteer would be treated as an employee too. It might be worthwhile asking for a copy of your organisation's procedures for managing volunteers which should include the disciplinary procedures.