First of all, the employer should have given the employee some training in operating and maintaining the mower, which does not appear evident. Without specific training and instructions, such as checking oil level or whether or not to attempt cutting "8 inch tall grass", any damage is on the employer.Was the employee expected to be an expert in the workings of the mower and paid accordingly?
In the end it depend on the diagnosis of what caused the failure or malfunction and how it could have been prevented. If it is determined to be operator error, was the operator trained accordingly to avoid it?