L.A.
You could place one of those plastic chairs in the tub. Let her cast leg, enclosed in a trash bag, rest on the edge of the tub. Use a hosed shower head to bath her and rinse her. You can always go back to the regular shower head when the cast is off.
Poor baby girl..
Here is an example http://www.homedepot.com/p/Delta-7-Spray-Handheld-Handsho...