I don't know how an auditor can be requesting that plant 1 be certified a standard, that's you as the customer that makes that decision, it's your requirement, not the auditors.
How you choose your suppliers/manufacturers is your choice. However the rationale behind how/why you use and have approved that manufacturer needs to available. As MIFSQN noted, you need evidence that the film layer complies to your specifications, is food safe and meets the relevant standards as well as your requirements.
Your manufacturer should be able to provide you with all the required supporting documentation to show that the packaging meets the requirements for direct food contact. You may have also audited the plant and have put in place your own specific requirements for the packaging film. We have a preference for our vendors to be GFSI certified, if not, we require a minimum of HACCP, we don't make our vendors get GFSI certification (we'd like that they do.) As long as they have HACCP and can provide evidence for the food safety aspects of their packaging that is all that is required.
When we get audited by our certifier and customers, they usually ask to see the packaging food safety compliance and that the vendor has a quality certification in place that our procedure states that they must have,nothing more, nothing less.