BELIEVE ME, I am trying to think of any way I can get away with carrying a good sidearm over there, any the bottom line is, it just isn't gonna happen. Difficult to take in, pretty much impossible to bring home, and while you're there, even if YOUR CO says he'll look the other way, one day you'll bump into a division staff officer or SMAJ who will let him AND his chain of command have it. A butt-chewing for him, and negative paperwork in his CO's file, MINIMUM. They know this and their careers aren't worth it. (Even if every single one of them would like some better weapons than those made by the cheapest bidder.) People are even getting UCMJ action against them for using unauthorized body armor, even if it's much better than the issued IBA.
The field I work in is kind of grey. Some of us will get attached to units that operate on their own, or 'OGAs', (Other Governmental Agencies) where it will be part of their job to not reveal to the enemy exactly who they are. Some of these guys have worked with these guys, know they will work with them again, and are taking personal weapons completely at their own risk. I asked them, they said, "Wait until you get there, see what the climate is, and see what everyone else is doing. Downtown Baghdad, no way. Long-term isolation out in Afghanistan, maybe."
What he will more likely find, is that when he gets there, if he asks the armorer for an M-9, he will probably give him one without a word. The main thing they are concerned about is making sure they have plenty of weapons to go around. If they have plenty, (And from what I hear, no one there has any kind of weapon shortage,) there is no reason not to let him have one. In my unit, it's NCOs get sidearms. After they draw theirs, everyone else can ask for one.
Having said all that, while every weapon should be kept clean, a pump gun is probably the best possible option for this environment. Just wipe it and keep it damp, and it will go forever.