In this situation the 28 gauge, a particular favourite of mine, is not a better choice if she intends to shoot waterfowl. I have never seen 28 gauge non-toxic in Canada. Even standard 28 gauge can be hard to find and it's expensive when you do unless you reload.
The right choice is a 20 gauge semi-automatic, a Remington 1100 or Beretta 391 in one of the reduced length/youth versions or the Browning Micro mentioned above.
28 gauge, you don't need to convince me of the delights of the 28 gauge. I've owned a bunch of them, two of my 6 shotguns and my only subgauges are 28s. But I think we do a real disservice recommending it when it isn't suitable for the purposes.