USA, Canada / Changing plane in the USA without USA Visa [13]

Hi, my Polish fiance lives here with me in Mexico, we are planning to visit Poland for xmas, she doesnt have usa visa and i do have it, my question is if she needs a usa visa only for changing planes in the states (we want to take that way coz the tickets are cheaper than flying from here to Europe) i will apreciate your answer,

Greetings!