Smuggling a $10K piece of jewelry into the US will get you more than having the item seized and forfeited. It will also come with a fine which is some multiple of the duty. That is all on you because you are the smuggler. Who owns the item is irrelevant.
Packing a $10K piece of jewelry in your checked luggage is a not very smart thing to do. It's one thing to have your suitcase go missing with a dirty shirt, another with a piece of jewelry.
If you do agree to transport the item, get an estimate of the value and have your friend prepay the duty (to you) so that you can properly declare it and pay the duty on arrival. If this is a valuable piece, CBP will want the appraisal and that is what you ought to use.
As to the making a false declaration about the whereabouts of the item, people who travel with valuable jewelry which was either purchased in the US or has previously been declared and dutied, have either completed a Form 4457 prior to departure or have an appraisal showing that it took place in the US for prior to the date of departure. Making a false statement is punishable in addition to the above by impriosnment for up to 5 years. Not that as a first offender you will go to prison for 5 years, but nonetheless not the kind of risk most people take.
Before making a decision, why not simply lay this out for the friend and have him do the leg work as to what the duty will be.