I agree that she probably is right that she won't enjoy it, so not taking her would probably be your best option. However, if you really want her to come:
-Onsite, the Carribean Beach Resort is reasonably priced, and has 1 pool for each 'island' so there is some chance of a fairly quiet time as long as you dont go to the main pool near old port royale, and she could do this during the day whilst you are enjoying the parks. There is also a small island in the middle of the lake, and a reasonable path for gentle walks as well as bikes available for hire and boats on the lake. These could keep her occupied during the day whilst you hit the parks. Definitely avoid the all-star resorts etc., as even the pools here get so busy during the afternoon that she probably wouldn't enjoy them.
-In the evenings, meet up for meals, and then see if she can be persuaded out to experience the parks then, when they have calmed down and it is not quite so hot. This would mean that she wasnt forced out into the heat and crowds, but you still got to do some things as a family.
-Typhoon lagoon is the least busy waterpark, and if you arrive early you can get beds in the quieter areas (although don't go at the weekends as it is much busier). Dont go for the ones near the main wave pool, or a big attraction, I think there used to be some that were quite quiet near the shark diving tank thing (although I could be way-off). If she intends to ride anything here, do it early morning before the crowds pick up. Castaway creek is especially nice (and almost empty) in the first hour after opening.
-If you take a car, perhaps you could also make an excursion to somewhere non-disney that she would like to go (a nice beach or something (I always go for intensive disney holidays, so not sure what the beaches around there are like, but am sure others will be able to suggest something).
-World showcase at epcot is also not stupidly crowded in the mornings as soon as it opens (11AM?) and might be something different for her to do one day.