If it is not possible to resolve the issue via an informal discussion, you should invite the volunteer to a formal meeting to discuss the issues with you and your line manager.
Os nad yw’n bosibl datrys y broblem trwy drafodaeth anffurfiol, dylech wahodd y gwirfoddolwr i gyfarfod ffurfiol i drafod y materion gyda chi a’ch rheolwr llinell.