There used to be a website called "Thrones of the World" that listed all the WDW restrooms. Apparently, it's no longer around because I couldn't find it. If anyone knows where to find it, maybe they will post.
Anyway, the restrooms are shown on the park maps with a little icon. None of the maps I found online (including the official Disney site) were detailed enough to show the restrooms. Trust me that the actual maps do.
All of the restrooms have handicapped accessible stalls with raised seat toilets and grab bars. At MK and Epcot, most of the stalls are
not large enough to bring a wheelchair into the stall (if you can get it in, it's very tight). The bathrooms in The Land at Epcot are not even big enough to get a wheelchair easily into the bathroom. At the Studio and at AK, most of the accessible stalls are large enough to actually bring a wheelchair into.
In case she needs to have a stall big enough for a wheelchair to get into, you will need the Companion restrooms. Here's a
link to the DIS general disability info page , which has a list of Companion Restroom locations.