Originally Posted by
tfar
It leaves him only with duty free if he has a direct connection home from FRA. If he has to change in the US, he won't be allowed to carry it on the domestic flight even though he just carried it over the Atlantic. Such is the logic of Homeland Security.
However, usually one has the opportunity to add it to your checked luggage (assuming one has checked luggage) after clearing customs and before dropping off the bags to continue their journey. In fact, there's often a sign and/or verbal instructions to do just that.