Transit stop in Germany on route to USA from Turkey with expired Turkish visa

1 - Turkey allowed him to leave, so Germany has no jurisdiction on that.

2 - Germany has no business if his Turkey visa was expired or not. They cannot penalize him or hold him for something against Turkey, unless he broke some regulation of Germany.

3 - Germany has to check only 2 things:

• As per German regulations does this person fall under German Regulations or not? i.e. if as per his passport he is required to have a German Visa or not? Which I doubt is required.. If at all he has broken some German regulations for TRANSIT?

• Does he hold a valid US Visa or not as they have to check that before allowing someone to board for the US

4 - If at all this person has been detained/ held back in Germany, then they need to tell him which German or US regulation is causing this.. and if they continue to be rigid, he has to call their local NZ Consulate to help remind Germany that they have no jurisdiction on transit passengers if they hold valid visa for destination.

PS: This is from my understanding & 10/20 years of traveling across US/ EU/ Asia. I wonder if there was something I missed and do not know about here.

Without details, it's difficult to understand exactly what's going on.

Preventing someone from boarding a plane to some third destination would seem unusual, lest there is some arrest warrant against him or something. Even if the German border guards are suspicious of your friend's real intent based on his infringement in Turkey and want to ensure that he does not remain in Germany, I would think that they would typically detain him until the departure of his onward flight instead of going to the trouble of sending him back to Turkey.

But the airport is without any doubt entirely under German jurisdiction, even the “international” transit area, and border guards do have the power to refuse entry, detain and forcibly remove any third-country national who presents himself to this airport, following the rules defined by German law. In particular, they don't need to find him guilty of breaking a specific rule or of some earlier infringement, a “reasonable suspicion” (begründeter Verdacht) that he is deceptive about his intent is enough.

He should go (if possible) to the New Zealand Embassy in Berlin. Or may be the Auswärtiges Amt can help him.

Hope this helps.