Since you bought the boots at a local shop, do they have a fit gaurantee? The shop I work at does. I'm not sure how bad your wife's foot issues are, but there are a few things that are easy and then some more that will be harder/require some expertise. What exactly is the difference in foot size, half, 1, more? Most people feet are not the same size.
It sounds like the boots are too big. Could be length, volume or a combo of the two. First thing, if the shop has a boot fitter (generally a ski boot fitter), go to them and they should be able to make things work. Ideally it would be free or cheap (see fit gaurantee above).
If that isn't an option, you might need to do some things yourself:
1) Your wife (and you) need some kind of aftermarket, supportive insole. I have the Superfeet Wintergreen (now they have Hot Pink for the ladys) in my snowboard boots. You just trim the toe of the insole to fit the boot and you are good to go. There are other brands out there too. Just make sure they are supportive, not Dr. Scholes gel. Even better would be to get some type of custom insole. Superfeet, Instaprint and Surefoot are a few brands.
Insoles will make a huge difference in stabalizing the foot and can be used to take up some volume.
2) Use the J Bars mentioned above.
3) DO NOT crank the boots tight, that will just cut off circulation and cause numb and/or cold feet.
4) There are boot heaters that you can buy to keep the feet warm. They are usually used with ski boots, but can easily be used with snowboard boots too. Burton even has a boot this year with the heating element built in.
Hope this helps. In the future, fit the boots to the smaller foot and then make the other boot a little bigger if needed. When you heat mold the shop can put a cap over your toes to add extra space, do that on the bigger foot side and not the small side.
One other thing, it was mentioned to tighten the boot with your weight back/on your heel. It is actually th opposite, you should put the boot on and then flex forward a few times to get your heel set back into the heel pocket. Then tighten. Remember, you ride with your knees bent. The right boot fit (out of the box) should have your toes touching the front of the boot when you stand up straight and your toes pulling back when you flex like you are riding.
How's that for a first post?