I'm looking for the specific story about the girl who was so sick her limbs were like twigs. How did Udi help her?

That moving story is detailed in a footnote within Chapter 33. The daughter of Khimji Lalji Joshi was gravely ill, and her hands and feet had become like twigs. Her mother first took some Udi from R. Hari Sitaram Dixit's house, which gave the girl some immediate relief. The family then took her to Shirdi, where she was carried to Maharaj. As the thirty-third chapter recounts, Maharaj applied Udi to her and blessed her, enabling her to walk on her own in just a few days. When she still had trouble getting up, Maharaj gave her a fruit to eat, which completely removed that difficulty as well.