It depends. If they scan the passports when they enter the U.S.A. and then come back the Canadians will see that she was illegal for 4 years and they will almost certainly not let her back in. If they go across the border without scanning the passports or recording their entry then she might be ok.
Now my friend and his wife want to go to the US for Christmas. Will there be problems crossing the border into the US and coming back again?