The upper 27 is fairly straightforward in my opinion. Wild Rivers Expeditions there out of Bluff, UT will run that section in their dorys at well below 500 with no problems.
You really only have one significant rapid on that part of the SJ, 8 foot rapid which a pretty easy move, especially with a scout, and for an experienced boatman. I would foresee your bigger problems just getting stuck on sandbars and such up around Chinle Wash.
Personally, if she wants to take the Dory, and she has the skill I would think it wouldn't be a problem. We have run it in loaded rafts down to the 300's with no issues, so as long as you are above 500, I would think you would be fine.