If hes smart.... I mean realllll smart. He won't fuck around with it. Attempting to take weapons parts back is bad juju. But little shit like a front sight post/gasblock.... theres a good chance it could make it through no problem.
But... if he's just smart enough to be bad for his own health his best bet would be to just toss it in a tough box in one of his units baggage containers and then get it when the container arrives back in the states. Having no first hand knowledge on unit deployment procedures or customs operations I can't gurantee this, but rumor is there's a damn good chance it could be gotten away with.... a friend of a friend of a friend might have gotten some war memorabilia back that way. Tell him to make good friends with his Unit Movement Officer and ask if customs is going to x-ray everyones tough boxes or just look through them. The machine sees all... just saying. Some units get trained inspectors from neighboring units "Army Pre-Clearance Program", others use the Navy Customs. Navy customs has all of the high tech gear to snoop through your shit.
Generally Officers don't do shit like that.