In my opinion, a CB auditor, assessing a system against ISO 13485, could have a valid need to peruse an FDA audit report. ISO 13485 emphasises regulatory compliance over customer satisfaction. An FDA audit report provides important and relevant data concerning a QMS ability in delivering regulatory compliance. So, there might be valid reasons for a CB auditor to be interested in seeing it. For example, to see if the management reviews are addressing FDA concerns.
But what would be an objection to share regulatory audit results of a QMS that is supposed to deliver regulatory compliance? If you and your CB have a common goal of strengthening your QMS, I don’t see what the problem would be. If you and your CB have antagonistic/adversarial goals, it is time for one of you two to re-think the relationship.