Towed Vehicle Causes Damage—Which Policy Provides Coverage?

Our insured is a towing company. It has a standard business auto policy with a garagekeepers endorsement and the covered auto symbols are 7, 8, and 9 for liability coverage. Out insured was hired to tow a vehicle and in the process of towing the vehicle, the vehicle being towed struck a parked car.

Would the liability coverage be provided by our insured's policy? Or, would primary coverage come from the policy for the vehicle being towed?