First, I certainly have NO sympathy for someone messing with KP. None.
I also have no trust of the IRS.
With that said, since they seized the computers, it naturally follows that this was done via a warrant. And the warrant most likely specified what they could search for, such as tax or financial records, common file formats that contain such records, etc. They had no authority to search for the KP, hence the claim of inadvertently discovering the KP. If they truly discovered it by accident, it's probably admissible. If they struck out on tax issues and decided to see if they could find something else, probably not.
If the guy is innocent, he should hire a top-notch computer forensic examiner post-haste. If the images were planted, unless the planter was way sharp, the examiner can probably prove it.
MM, CCE (Certified Computer Examiner)