i dont know exactly what he will construct in CAD and no idea bout specific aircraft piece, but i think that is too serious, youll need a Quadro gfx cards instead of gaming gfx cards, most of CAD use opengl instead of Direct x.
take a look at this
http://www.tomshardware.com/reviews/best-workstation-graphics-card,3493-13.html
for proc he will need a Intel Core i7-4960X Extreme Edition 3.6GHz 6-Core Processor
its about $1,200.
for RAM he will need a total 32Gb which is $440.
and he will need a lot of storage maybe samsung 840 512GB SSD and a 2-3TB for HDD.
for psu i think 750w is enough .
for Monitors > (2) ASUS PA279Q Black 27" 6ms WQHD HDMI Widescreen LED Backlight True Color Professional Monitor 350 cd/m2 100000000:1 Built-in Speakers height&pivot adjustable $1,600 ($800 ea).
and that is too expensive
but of course your budget... i highly recommend to get i7 and GTX Titan Black with 16Gb(but i dont know if this rig will able to give him a performance that he need in his work, because its too serious) for game it can play all games(even 2015 games) in ultra settings 60+FPS
im not well known about constructing a serious CAD rig for professional skilled CAD operator
im sorry if i cant help