Last night I was logged in using my T and could only write with a mouse one letter at a time.
My answer (NO) was based on the fact that your nephew is already living in Argentina and even if he has permanent residency (according to the info in this link) he would need to have owned and used the car for at least three months before it arrives in Argentina.
https://www.angloinfo.com/how-to/argentina/transport/vehicle-ownership/importing-a-vehicle
This link does not include any information as to how long he would have after receiving permanent residency to bring the car into the country.
So, if he does not yet have permanent residency he can't import a car duty/tax free, but he might be able to if he leaves the country, buys a used car with a value of less than $15,000 USD, and uses it for three months prior to getting permanent residency.
I wonder, however, how it would be possible for a foreigner to do that in such a short time span. Returning Argentine citizens must have been out of the country for two years to import a car duty free. Perhaps the law actually works in favor of the foreigner in this case, but I doubt it.
"The vehicle may arrive in Argentina no more than three months before the owner's arrival in the country and no later than six months afterwards."
If your nephew already has permanent residency and has been living in Argentina for more than three months it simply would not be possible for him to leave, buy a car, use it for three months and return to Argentina within the allowed six month time frame.
If he only has temporary residency, perhaps he could pull it off, but only if he can spend three months using the car in the US while he applies for and waits for his permanent residency to be approved and the time previously spent as a temporary resident doesn't count..