This seems like a problem with your friend moreso than with 3D printing in general. Most people I know who hear about 3D printing don't immediately think of making weapons. Toys and weird gadgets tend to come to mind first, or maybe an office accessory like my laptop stands. The fact that your friend immediately jumped to the conclusion that it's for making weapons says a lot about the way they think about the world.

I agree that the law seems to validate the viewpoint, but I disagree that it's a common one, nor that you should have had to spend time building that trust.