Happy ending -- Armalite replaced the complete lower assembly at no charge. My friend's AR-180B is now in pristine condition, but he has no intention of shooting it. It will be traded for an AR-10. Since the 180's failure, he bought a Bushmaster M-4 style AR in 5.56, and wants a companion rifle in 7.62. I'd still prefer a pushrod-operated self-loader, but my friend is a long-service Army vet who lived with the direct-impingement system throughout his service, and he finds it reliable. He did, however, leave the Army before the current Mideast wars, and has never carried the rifle in a sandy, dusty environment.
I have heard that some Special Forces personnel are equipped with the new HK416, which does use a mechanical connection between the gas port and the bolt carrier. It is interesting that nowadays some units are equipping themselves with non-standard weaponry, such as the USMC buy of Kimber 1911s.