Also, strictly as a combo 3D-printing and engine enthusiast: Never with a GUN to my head would I install 3D printed parts in a CAR engine, let alone in an aircraft engine. This is spectacularly poor judgement on the part of the owner.
Rocket engines can be 3D printed, in fact there are some engines that can only be made using that kind of technique due to internal structures.
The printing is the easy part.
The extensive testing and validation that it will actually work as intended and in your situation is the hard part.
Skip that hard part, especially for anything that flies, and you are risking lives, both those in the air and on the ground.
Seriously, just because the specs on the label say X and other docs say the running temperature is Y, does NOT mean it will work. Take the measurements in your situation, test the thing extensively on the ground.
Then, maybe, it'll be worth flying. Or, you'll be there after some hours of testing saying: "good thing I didn't try to fly with this", and still have a usable aircraft.
Edit: missing words, clarity.
The fabrication technology doesn't matter. The qualification process, on the other hand ...
This is the primary reason why I never got a pilot's license. I suspect I would spend far too much time making sure the maintenance was up to standard and far too little actually enjoying flying.
Well, yes, but... In this case the fabrication technology and the lack of qualification process likely go hand in hand. They wouldn't have a qualification process unless they were manufacturing enough of these that plastic 3d printing wouldn't be cost effective. The shortcut is the point.
The shortcut is to ask the mechanic to come for a test flight after repairs. The place I learned to fly was owned by a mechanic, and the daughter ran the flight school. Given that the daughter might be test flying the mechanic's work, I trusted him to keep his planes in good shape.
Or do they get their parts from some vendor at a swap meet who spends most of his time fiddling with his Ender 3?