| Unfortunately, that type of device is just not possible. Sound travels in air as a compression wave, but if you want, you can think of it like a wave in water. There are peaks and troughs. You can also stack waves on top of each other, like little rippleson big waves, for example. Earphone/headphone based noise cancellation devices work by generating the exact opposite sound wave (a peak where there is a trough, and vice versa), which, when added to the incoming sound wave, exactly cancels it out.
Unfortunately, there are only two places where this can work. Either at the source of a sound, or at the receiver of the sound. Think about dropping a pebble in a pond. It makes a series of rings. If you drop another pebble in a different spot, in some locations on the pond, the ripples will be smaller. In others, they will be larger. There is no way to cancel the sound waves over a large area.
The best that she can do is try to block the windows so the freeway sound does not enter as loudly, buy a white noise generator and try to drown out the cars, or just plain old get used to it. I lived in an apartment near a freeway once, and it took about two months before I reached the state where the traffic noise was no longer noticeable. | 
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