The issue you'll need to address is whether you'll want to keep the Canadian plan on the phone for the months she is in the states. You can flip it on and off, but you'll lose rollover minutes when you take that approach. You'll also save $20 a month.
You should remember that ATT has had a mixed history on their Canadian plan. They had it, then they took it away, then they brought it back with a small pool of Canadian minutes, and then they opened it up. You should consider these things when deciding to flip on and off the plan. If you look at the Verizon's forums or Flyertalk's tech forums, you'll see a lot of people who permanently lost their international data plans when they temporarily turned them off. International plans are changing quickly. If there is one you like, it is often worth keeping it even if there are months you don't use it. I pay $64.99 a month for my iPhone International data plan and there are months I ask why. I just returned from three weeks abroad where I went through nearly 800 megs of foreign data and respond "that is why."