Dringge, in what part of Northern Minnesota is your daughter (without getting too specific)? We don't have much in the way of bow-making trees in the Red River Valley area, so if it's not too far east, it would be fun to make a day trip and try to find some buckthorn to harvest.
Before I do try to find some, I saw the Minnesota DNR website says it's illegal to import, sell, or transport buckthorn in the state. I'm not sure if they're just referring to live plants or any part of the tree. I'll check in on Monday and post the answer here, as I wouldn't want to get on their bad side.