Do you get the joint in the duty free shops? Asking for a friend.
The problem that you might have in Jersey is if you have to take possession of your firearm and suppressor and leave the airport. Then you are in violation of their laws. As long as you are in transit and your luggage is checked through to your destination where you never take possession then you are fine.
For the joint, I suggest Schipol would be your friend's best bet!
As for luggage checked through to a final destination, again, I think that depends on local laws. Many have found, for example, that even though their luggage is checked through Doha to JNB, the Doha authorities sometimes take an overly keen interest in the firearms.
And, again, in Germany and Holland, if you don't have the proper paperwork to transit the country with your firearms, they will go into the luggage area and take them and then find you.
I have heard of people who entered the US on an inbound international flight and were connecting to an outbound international flight and thus never clearing US Customs being arrested for having contraband in their luggage or being otherwise inadmissible into the US. In these cases it has never been an argument that you were not intending to enter the US or your baggage wasn't going to stay in the US. You are on US soil and US laws apply to you regardless of how little you want to be there. I have very little doubt that a person who is not otherwise admissible into the US will not be allowed to transit the US, no matter how little time that transit takes.
Lastly, I note that if you are flying to Zimbabwe and have checked your firearms straight through to some airport in Zimbabwe, if you are transiting almost any airport in Canada or Europe to get there, you will have your firearms seized, regardless of of their being checked through, as they enforce a UN embargo on firearms to Zimbabwe.
The only safe assumption is that so long as you or any of your baggage is physically in a country (regardless of where in that country you may be, for what purpose or for how long), you and your baggage are subject to the laws of that country.