Pollution Clean Up under BAP

Our insured is a long haul trucker insured under the business auto policy (BAP) with a truckers endorsement and a broadened pollution liability coverage endorsement. The insured is in possession of a nonowned refrigerated trailer, completes a delivery, and before returning the trailer to its owner, the insured disconnects the trailer for the weekend. Over the weekend, the refrigeration unit on the trailer leaks diesel fuel.

Does the insured's policy cover the pollution clean up? We think that the spill was due to poor maintenance by the trailer owner.