Does he want it to look like an M-16 or an M-16A1? The M16 had no forward assist, m-16A1 had the forward assist.
The civilian version of the original M-16 was called the AR-15 SP1 by Colt when they were offered. They had triangular handguards, a shorter stock, fixed sight bridge with L-shaped rear sight adjustable for windage (flip up to switch from 300 yds to 500 yds sights), and a very thin pencil barrel with a 3-prong flash suppressor. There are outfits that offer the smooth-side receivers, but I don't know if anyone offers a complete rifle with all of the correct features. Others on here will know.
The M16A1 used a teardrop shaped forward assist, had no case deflector lump, had triangular handguards and shorter stock, had a fixed sight bridge (carry handle) with an L-shaped rear sight adjustable for windage only (flip up to switch from 300 yds to 500 yds sights), and a cage flash suppressor with vents all the way around. All of these features are available as parts, again I don't know if anyone offers a complete rifle with these features.
So, long story short, if he wants an AR15 that looks just like the M16 he carried in VietNam, he may have to build one.