Can you recount the story of how Udi helped Khimji Lalji Joshi's very ill daughter?

Certainly. Chapter 33 includes a footnote from R. Hari Sitaram Dixit detailing this event. Khimji Lalji Joshi's daughter was ill for many days and became very serious. Her mother went to Dixit's house, took some of Maharaj's Udi, and the daughter experienced immediate relief. Subsequently, the family took the girl, who was too weak to walk, to Shirdi. Sai Baba himself applied Udi to her and blessed her. Within a few days, she began to walk on her own. When she still had trouble getting up and sitting, she complained to Maharaj, who gave her a fruit to eat, which completely removed her remaining difficulty.