Laurie... I believe she's going to need SOMETHING proving that she has permission to take her daughter out of the country. Maybe she could get a judge to sign it, instead of the father, I don't know. But it won't be much fun for any of you if you get to the port and are not allowed to bring her. I took my two sons when they were 4 and 6 to Canada with my mom. They almost didn't let us in because I didn't have a notarized letter from their father. They talked to me and to the boys separately and then let us in. I don't travel without a letter anymore.
Don't take the risk. Here's a link to a permission form that's helpful.