I agree with ThinkTink. Out of Sept., Oct., Nov., and Feb. I found Feb. to be the least crowded.
I would also caution against Sept. because the free dining offer has really increased the crowds.
Couple of other tips for avoiding crowds. If you are not renting a car, then use a car service like Quicksilver to get you to your hotel. Stay at a hotel like Coronado Springs that is very big and spread out. When we were there it was nearly full and we felt like it was empty. Also since there tend to be conventioneers that are not visiting the parks their busses are less crowded.
If you go ata slow time of the year and want to see a show, it is likely that they will not be full, so take the chance and hang back from the line and go in just as the show is starting.
If her phobia is documented you may also want to check with guest services to see if they can make some accomodations for her problem.
Good luck!