He needs to talk to a expert in the field, not ask on an internet forum
Who can he ask or where can he find out?
By searching on the internet.
someone asked us, he is traveling to Europe is a dual citizen of both countries. When he leaves the USA, does he have to declare the gold jewelry? (maybe 1000 pieces)
Doesn't anything over $10K in value have to be declared?
I think that’s only cash or monetary instruments
That sounds like it's being taken for trade rather than personal use. I would assume a quick call to a customs broker might help answer the question.
do you have a number?
https://www.cbp.gov/contact/find-broker-by-port/4701https://www.trade.gov/customs-brokers-and-freight-forwarders
this looks like it's for imports.