not worth cutting down and threading your original barrel since itll be a 2 3/4" chamber. i realize that the gun is also a 2 3/4" gun, but there is no point investing money in a 2 3/4" barrel. a 3" barrel can be used on that gun no problem, and later on he can even convert the gun to 3" by swapping the ejector.
the 18-20" requirement is difficult because the only barrels that come factory threaded in that length are the 870 Tacticals - and most people buy those guns specifically for the short choked barrels so you wont be seeing many on the EE.
if he wants a semi-short tube tapped for chokes his best bet is to watch the EE for a few weeks and pick up a short turkey barrel (21-26", ~$150). or get an 18" barrel and pay the $100 to get a smith to tap it for chokes (~$150+100+price of chokes, not cheap).
easiest solution is probably to buy one longer barrel with rem-chokes for hunting (28" VR take-off barrels are a dime a dozen), and just cut down the existing 2 3/4" Wingmaster barrel yourself to 18.5" and install a bead.