Had he reported the full amount, there would be no issue...
There is no limit to how much currency or other monetary instruments travelers may bring to or take out of the United States. However, federal law [31 USC 5316] requires travelers to report all currency of $10,000 or greater to a CBP officer and complete U.S. Treasury Department Report of International Transportation of Currency or Monetary Instruments [FINCEN 105].
https://www.cbp.gov/newsroom/local-media-release/cbp-officers-seize-33k-unreported-currency-egypt-bound-man-dulles