I had a break very similar to Lisa (Talking Hands). Totally shattered the ankle. I didn't manage to get to Disney during that time but being non-weight bearing for three months was pretty tough even without a Disney Trip!
If she has a shower chair, as Lisa suggested, using a plastic garbage bag and some duct tape you can keep the cast dry in the shower. My husband used to wrap me up every morning before my shower. He did have to help me in and out of the tub. A HA shower would be much easier to deal with!
When you rented the
ECV did you ask about having it able to elevate the leg? The ankle will swell if not elevated and will be very uncomfortable. I am not familiar with equipment that comes with a rental ECV but most of them I've seen at the parks don't look like it would be easy to elevate the leg.
If she is taking narcotic pain meds, she might want to avoid anything that spins! Even when I don't have broken bones I sometimes take pain meds for arthritis and some other stuff and they don't mix well with Mission Space for me!
I did do a trip in a manual wheelchair last May. I had a stress fracture in the foot with the bad ankle. I walk a bit "off" because of the ankle and it caused the damage. So my daughter pushed me. This saved the expense of renting a
scooter since we own a wheelchair. Personal chairs are much better than the Disney rentals I understand. The cast members were great when they saw us coming. They quickly directed us to where we should go when you reach the point of the line where they have to separate wheelchairs from the regular line. Not sure how many are in your party but we had three and they always kept us together. I did not get a GAC since the wheelchair was the only accommodation I needed.
If her cast is high enough that she can't bend her knee (mine was mid-thigh) it might be difficult to board some attractions.