What you are seeing is a difference in reflections, not a color mismatch. Unfortunately there is very little if nothing at all that you or your bodyshop friend can do about it. Paint fades under UV with time, specially worst with the metallic tones, so to match that degrading effect one would need a type of skill set that unfortunately nobody have developed yet. For a 100% match you'd need to respray the entire side of the vehicle.
Its one of those thing... if you wouldn't have told us we would have never found out
Nice job from the look of it if you ask me