I would take what I already have for the most part; a BM Dissy and both my 1911's, in case of catastrophic failure. In fact, I think I'd also buy a back up Dissy (fully set up exactly as the first was) as well and, of course, run at least 1K rounds through it first. After that, I'd start shooting without cleaning so I would know how many rounds the gun could take before malfunctioning due to needing to be cleaned. Obviously it would have to be modified for the nasty sandy conditions in the box, but I think it would give a good baseline.
I would definitely throw on either an EOTech or an Aimpoint as well as both a good single and double point slings (I shoot lefty so the 3-point is out for me) and maybe look into some of the other options like a FF railed forend, light and VFG. Seems like the more rounds I put through my Dissy the more I'd actually appreciate some of those things, but it's taken me quite a few thousand rounds to figure that out.
Couple other things I'd add, at least double the number of mags (GI for the AR and Wilson for the 1911's would be my choice) that I can carry with me on a daily basis, exras left with the backup rifle/pistol combo; again just in case. I don't know exactly what their jobs require of them, but if they can be loaded out with a vest and everything constantly they might even want to look into a full backup of everything kit, but then again that is probably overkill for a non-military/shooting contractor job. I don't know the first thing about tac-holsters, but the vest-mounted ones look good to me. If they have extra stuff when they leave it can probably be sold to others over there or back home. Good sun and clear-lensed glasses, gloves (something like a good pair of Nomex flight gloves) and boots, all with backups. Maybe hit the Army/Navy Surplus store and try to find a used flight suit or two.
Training, training, training. IIRC, there's a guy on here who teaches a contractor-going-to-Iraq/Afghanistan course, that may be good for them.
Just for the record I'm not a professional anything, let alone shooter, these are some of the things that I would consider, take what you want, leave the rest as usual.