hmm i know with movistar you can use a phone from the US as long as it has a chip on it, they remove the one it comes with and put one in from that service provider. its not costy either, i think its about 100 pesos or so for the chip (its more like a mini sd card).
this is a prepaid so she will put money and she can call the US. for every 15 min its about 10 pesos or so, not too sure but its not bad at all. then ofcourse the rates for national calls. no money used if she gets incoming called either.