Canadian working in the US

Hi, A friend of mine is a Canadian citizen with US green card and only working in the U.S. (W-2) and has only been in Canada for about 1 week for a visit in the past year. He has no income from Canada. He filed taxes in the U.S. Should he file taxes for Canada as well? Should he pay taxes to Canada for the income he already paid taxes in the U.S? tnx