For starters that may be criminal fraud. Since he's defaulting not because he's bankrupt but because he just doesn't want to pay - he's in fact stealing, or at least committing fraud. That type of criminal offenses would be criminal in his home country as well (I am not aware of any country that doesn't criminalize stealing and fraud), and he may be prosecuted if the US government does file a complaint, even if he's "at home". I do not know of any country that limits its criminal code only to their own territory. He may also be extradited back to the US, if his home country has such a treaty.
That said, if he has no intentions to come back to the US now - he may change his plans in the future. It will be too late to "change his mind" then, he will be in big troubles, may be arrested on landing, denied visa, most likely denied any credit, and harassed by collectors.
Last but not least - the debt is a contract between him and the credit card company. Even if the US doesn't file a criminal complaint at home - the credit card company may file a civil complain and sue him for the money. The courts in his home country will likely to accept jurisdiction since he refused to return to the US.
Bottom line - stealing is wrong, and the victim ( the bank/credit card company) may very well go after him wherever he goes, and for a long time. Statute of limitations in the US doesn't advance while he's out of country, so the matter may still be open even if he comes back in 50 years, and the victim may also pursue action against him in his home country.