I'm afraid I've not read the responses to this thread yet, so I may end up repeating some people. Sorry if I do!
I reckon you could probably do Disney in July, as long as you manage everything
really carefully. You won't be able to do commando touring; you won't see everything; you won't be going all day from dawn to dusk, but you'll still be able to find loads of fun stuff to do! Do everything you can to minimise time outside and keep her cool (OK, that's rather stating the obvious, but I am working up to something here

). A few tips off the top of my head:

Use FastPass for any attraction with an outside / partially outside line - there will probably be a fair few, so be ready for a bit of waiting

If you want to watch parades, pick a cool spot and have someone in your party stake it out early (if she's using a wheelchair, the MK wheelchair viewing area in Liberty Square is shady and cool, but I'm not sure what the rules would be on someone else holding the place without her there)

Avoid being out in the middle of the day - hit the parks at rope-drop, then head back to the hotel when it starts getting too hot (allowing time for waiting for buses etc). You can go back out and explore the parks in the evenings. If she's not doing well at all during the day, go nocturnal and play in the parks when evening rolls around!

Rig up a cooling system on her chair. you can get small battery-powered fans that can clip onto the frame, ice-packs / damp washcloths etc around her (though obviously make sure she doesn't get too cold) and some kind of shade. You can pick up a parasol in Liberty Square that will clip onto a wheelchair or stroller frame, which would also look pretty

Know where you're nearest 'cool place' (shops, shows, restaurants etc) is at all times, so you can get there quickly

You may want to avoid Animal Kingdom, as this involves being outside in the heat
a lot, with longer walks to places, more open air theatres and outside queues

Tom Sawyer's Island is probably another 'no', but as she'll be touring in a chair, she wouldn't be able to explore there anyway

EPCOT FutureWorld might work well for you, as you only have to go outside to get between the pavillions, which means nice long cool down-sessions

Plan ahead - you probably won't be able to see everything while you're there, so work out before you go, what the 'must see' attractions are for everyone; you don't want anyone going home unhappy!

If you want to hit the water parks, pick an over-cast day and make sure she wears an old T-shirt over her swimming costume (the bigger, the better). We've done this since we were little to stop us getting sunburn, but it also keeps you cool

Keep your plans flexible, so you can take longer breaks, stay at the hotel for a 'down day', or go at a gentler pace as necessary

If she gets good days and bad days (I'm afraid I'm not familiar with the condition), you might want to save DisneyQuest for a bad day, as it's all inside and fully air-conditioned.

Keep hydrated!

Oh, and you may have to take a few extra ice cream breaks, but I'm sure she'll cope with that somehow
Basically, all this boils down to: Be sensible, be flexible and make sure she listens to her body. Do whatever you need to to have a safe and fun trip for everyone.
Good luck and have a great trip!
