Ok, than that is what you should tell your customer.
But I am wondering, will you not:
- ask the third party for a cause analysis/ motivation?
- include this incident in the evaluation of the third party and perhaps even reject this third party transporter (temporarily)?
- take action to the products delivered?
- implement inspection of trailers before loading? (if this is already there, check the inspection done, and speak to person that conducted the inspection. Why did he approve?)
- risk assess the incident to find out if you should indicate this in the contract with the third party transporters?