I agree that you'll get more info in the Disabilities board. Some things to let us know are what kinds of issues is she dealing with currently? How does she handle heat or cold?
We chose October for my daughter's trip, as I had looked at temperature averages, without looking at highs and lows. It was a bit warmer than she would have liked best, she doesn't handle heat very well. We just went in Dec-Jan and even though she doesn't handle cold well, the cold days were ok for her, and the warmer days were just perfect.
With a wish trip the crowds won't matter as much but you'll still probably enjoy it more with less crowds, so choose a less crowded time (as adults we really enjoyed the food and wine fest, and the kids loved the Halloween party--later october would have been a better bet for us temperature wise).