Wesley,
I went through this exact process last October with my naturalized Argentine fiance. We wanted to go to the U.S. for Christmas together. A passport is required at the time of the visa interview, so you need to get on that first. Yes, she does have to go to the U.S. embassy in BA for the visa. After she gets her passport she'll need to:
1.) Call at specific number at the embassy and pay U$15 for a PIN. Or she can go to a Pago Facil location and pay, in pesos, the same and receive a PIN. That will give her a time and date for her visa appointment.
2.) Before the appointment you have to pay the visa fee at a Banco de la Ciudad. In October it was about U$130 (if I remember correctly) but that may have changed, so check it out.
3.) Collect all the documents they might ask for. A basic list would include her passport, deeds to any properties/cars in her name, a school transcript showing she is enrolled, work contract showing that she is employed, and any documentation her parents have showing their ties to Argentina. If she's planning on staying with someone in your family that person should write a letter of invitation. Basically, you need to prove to the embassy that she will return to Argentina before her 90 days are up.
4.) Obviously, show up to the embassy the day of the interview. You need to be there about 15 minutes early. You, as a U.S. citizen, CAN GO IN WITH HER. Just be sure to bring your passport. That's exactly what I did and I was part of the interview and answered a lot of the questions myself.
5.) Answer all their questions honestly, show as many documents as they ask and hope you get someone nice.
6.) If she gets the visa (and you'll be told at that moment) she'll have to leave her passport at the embassy so that they can put the visa in it. It will be mailed back to her within a few days, so you'll have to pay DSL at the embassy as well.
Our experience was a very good one. Like I said, I was part of the interview and even answered all the questions in English. The interview took about 10 minutes and my fiance was given a 10 year tourist visa, even though he has no property or car and is working en negro, so he doesn't even have a legal work contract. We stayed at my mom's house, so she wrote a letter of invitation talking about how important it was for my fiance to come to the U.S. to meet our family and spend Christmas with us.
People might say that she shouldn't mention you or say she has an American boyfriend, but IMHO you shouldn't lie to the embassy. Also, my fiance didn't buy his plane ticket before the interview because we didn't want to pay all that money if he didn't get it. So, it is kind of a crapshoot, but it is certainly doable. All the information you need (phone numbers, etc) is at
http://argentina.usembassy.gov/non-immigrant_visas/how-to-apply.html.
Good luck!