As posted, CBP does not have access to credit card data.

That said, stores that sell high-end products to Americans have passed information to U.S. Customs since well before the days of universal credit card use. That hasn't gone away just because people don't use travelers' checks any more. This purchase is borderline for CBP to care, but "your relative" should assume they will know about it. It is simply not worth the risk to try to sneak it through. The downside is way more than the upside, saving what is - to "him" - surely a trivial amount of money.