I have imported a fair number things into the US. The harmonized tariff schedule is on line:
https://hts.usitc.gov. Do a search under
wrist watch and you should get to the heading for 9101. From there you will need to figure out the correct sub-heading and Stat. Suffix. But my initial read would that one could expect to pay 3.1% of the value. If they know exactly which watch they will be buying one can call the customs office and ask for a specific reading. They are quite helpful. I do this to verify duty and then print the appropriate pages for the goods. This helps whether the item is being shipped or hand carried into the US. because the customs office has the duty right there in front of them. This is especially helpful when there is no duty.
Of course given they are bringing the item in themselves, I would declare it, but then let the Customs office decide if they want to collect the duty. I.e. have the duty paper work ready to verify the correct duty is being applied (i.e. Customs may charge less or nothing). Also when declaring the item make sure one writes down the value - not the price paid as that would include the VAT. Finally make sure when leaving the country there is time to find and wait in line to have the VAT paper work stamped at the airport.