The first time I came down here in 2006, I brought traveler's checks. I had a hard time cashing them, and none of the apartments that I tried to rent would accept them. I finally found a bank on Las Heras that would cash them (can't remember the name, but it was French-sounding). They were American Express traveler's checks.
As far as buying them here - she'd run into the same problem - how to get the money to being with.
Sara, it doesn't hurt to have cash anyway, as long as you are careful with it. I'd pull the money out of the ATM anyway, just to be safe, in case there is some problem getting the American Express card down here before you have to pay.
For example, if it comes via regular mail, you may find that the mailing itself delays more than a month getting to you here. If you have it sent by someone in the States, have it sent by private mail such as DHL or FedEx. I recommend DHL because it's cheaper, although someone recently told me that DHL pulled out of the US for private consumers and just does business transactions now. I haven't verified this.