Your friend is implying (perhaps indirectly but the same nonetheless) that a mechanic, with no mathematical or scientific background, would be able to design a manned space vehicle. A mechanic can fix things, but how did they learn to do this in the first place? The Engineers do not just hand over a complex machine and say "ok guys, you figure this thing out." Mechanics are great because they can adequately service machines consistently.
However, does that mechanic have the knowledge to evaluate the geometry of the machine and construct shear and bending moment diagrams to find stress maximums? Can that mechanic use calculus to discover the area of a complex surface that is needed to find the area moment of inertia? Does this mechanic even know what a force vector is (the basis of all stress studies) and how to break it into components? I doubt it.
