US citizens and resident aliens are taxed on all their income, regardless of where they live. They may qualify for foreign tax credit or exclusion of earned income (from your post, sounds like they're retired). You might be able to pro-rate income for state tax purposes. If they owe Canadian tax, they need to know how much in order to calculate foreign tax credit.
Discuss foreign tax credit and or exclusion to both taxing agencies. US received all the 1099's, W-2" etc. how to calculate. What form does Canada use? They moved in August so about half of earnings would be US and half Canadian as I see it.