Bad customer Service

My brotherwanted to take his Opel Adam for a service which was still under the warranty agreement. Due to Covid he did not use the car frequently due to him being affected and not having employment When he arrived at the dealership they refused to assist him as they indicated that the time period was too long even though the car only reached the required kilometers at that time. I feel this is not fair in the Covid times as it was not his fault that he did not have employment to use his car during this time. Please advise how we can resolve this issue