Just something to consider - how will you plan on accessing the money from your rental apts? I'm sure you are aware of the challenges of the banking system here, you can't just transfer money into Argentina easily if you plan on setting up bank accounts, etc here. You can of course use ATMs but those have fees and you will get the official exchange rate (although not really an issue if you are withdrawing from a brasilian account with reals)
Actually, it is an issue if withdrawing from a Brazilian account with reais - it's exactly the same as withdrawing from a US account with US dollars, and just as expensive. The official real-peso exchange rate is currently 3.38, while the blue real is 5.10.
He has many options to withdraw his cash in foreign currency:
1. The main difference I guess is that it's much easier to fly to Brazil once a month than the US and bring the reais in cash (you don't even have to go to São Paulo, he could just fly to Porto Alegre, a 1.5-hour flight, or cross the border to Foz do Iguaçú). You can bring into the country the equivalent of USD 9,999 in reais, per person per flight.
2. Alternatively, you could also exchange your reais for US dollars in Brazil, and bring US dollars. Depending on the cueva, sometimes there's a slight difference between spread of the blue dollar and the blue real, but it's not really that significant (once you find a good and reliable cueva - since the blue real exchange rate is less widespread, it's easier for them to rip you off if you don't know what the day's exchange rate is).
3. You can also even transfer your US dollars to your Brazilian bank account (you will get reais). You can easily do this via PayPal, which works in Brazil just as well as in the US. This will set you back 6-7% per transaction (PayPal fee).
4. If you're planning to use Xoom, you could do the opposite: send your fiancée's reais to the US (via PayPal) and then wire your US dollars to Buenos Aires via Xoom. This is the priciest option though, for 2 reasons: 1. She'll need to use a Brazilian credit card to send the money from her Brazilian PayPal account to your US PayPal account, and she'll be charged "Imposto sobre as Operações Financeiras" (roughly 7%); and 2. the Xoom exchange rate is not nearly as good as the one you can get at the cuevas.