The problem is that it's probably a fairly heavily discounted business class ticket, which is why the change is so expensive. Unlike economy fares, where there are a lot of different discount levels, business class is much more limited, so once you're anywhere near the departure date, you're likely to have few options.

So I don't think colleague is likely to be able to change the EZE ticket for a reasonable cost, but is there any possibility he'd be able to buy a PIT-IAD roundtrip that would get him back just in time for his originally-scheduled flight? If so, he should be able to check in at IAD (including checked luggage) for the entire IAD-PIT-IAD-EZE trip. He might even be able to "bow out" of the IAD-PIT-IAD legs at that point, too, especially if there's any chance he'd miss the return flight.

However, he would have to be careful in that case that they don't invalidate the whole ticket to EZE due to him not actually being on the first leg from PIT. Best bet would be to check in the RCC after the PIT-IAD flight departs, and then again at the gate for the IAD-EZE flight, just to be sure nothing gets messed up.