It sounds to me from the scenario you out lined the ER staff thinks there is an osteosarcoma or something similar. Depending on how advanced will determine if she keeps her leg or life.
If you don't fully clean a deep wound, your presuming the limb must be removed.
Something in the story is wrong.
“Something in this story is wrong”
That is the problem. We cannot find out a true full story from the mother. I am hoping my sister can talk to Alyssa and find out more, but that won’t be until tomorrow.