Try Boot Gloves first, since they're only $29.95. They work pretty well for an inexpensive solution...as long as you put them on when your feet are still warm rather than waiting until they get cold.
If those don't work, I 3rd the Hotronics idea. In that case, you would then have both tools, if her feet are really stubbornly cold or for unusually cold days.
Boot Gloves are a waste of money...have to put them on when the boot is still warm and even then, they tend to trap ice and snow and only exacerbate the already bad situation.
Best overall solution are the Hotronics. Anything else will be a Bandaid solution.