Yes, if you find a mistake has been made then you file an amended tax return in order to fix the mistake. Apparently this was a complicated case as he was working for the World Bank at the time which is treated like working in another country but isn't another country. The CPA he had been using for years wasn't aware of the intricacies of this sort of case so he gave (in writing) bad advise. Later on when he switched CPAs the new CPA reviewed his other recent returns, identified the error, and amended returns were filed. I'm still not seeing a big deal here.