Can you recount the story of how Sai Baba healed the very weak girl who couldn't walk?

Certainly. A footnote in Chapter 33, noted by R. Hari Sitaram Dixit, tells the story of Khimji Lalji Joshi's daughter. She was ill for a long time and had become so weak that her arms and legs were like twigs and she was unable to walk. The family brought her to Shirdi, where she had to be carried and seated near Maharaj. Sai Baba applied Udi to her and gave his blessings. Within just two to four days, she regained enough strength to start walking on her own. When she still had some difficulty getting up, she told Baba, who then gave her a fruit to eat. After eating the fruit, her remaining difficulty was also completely removed.