I bet she would have a blast, but there are just a few concerns as mentioned above for low water MF ducky time: 1) even at the end of summer, cold water means she'd want to dress for it, 2) the vast majority of rapids will feel splashy and fun, but there are a couple -- luckily, easily walked -- that will look intimidating to her so as long as they are paying attention to the maps and have intention to scout, she'll be able to make that decision... I'm thinking Tappan Falls and some others in the series will look intimidating, although the swims are pretty clean and probably fun, 3) I haven't done much ducky time but my impression is that you can be swimming a lot in certain rapids. No big deal but helmet and knee protection and a willingness to be careful of rocks will help.
The low water MF affair is, despite the technical spots, a pretty slow water affair. Lots of time to catch your breath and avoid hazards. And a ducky will be able to take the coolest little lines!