How many people had access to the customer's computer? Did he buy it new, out of the wrapped sealed box? If there were more than one person with access, how would it be determined the correct person was being pursued?
If you gave a schoolmate of your kids a ride home, and unbeknownst to you or your kids, a residued crack pipe came out of the school kid's pocket and wedged in behind the rear passenger seat, you'd love having the cops charge you with possession.
Sometimes such calls to the cops make big media splashes. If the wrong person is pursued, how is that bell unrung?
On the other hand, if the customer HIMSELF did deliberate purchases, downloads, storage, and some prior owner, guest, prankster or cookie combo had nothing to do with it, you face less of a dilemma.
And if the weird stuff was wiped clean from his system and hard drive, what recourse could he claim against your shop?