I heard a story about a skeptical visitor to Shirdi. What happened to him, and what was the role of the devotee Tarkhad?

The skeptical visitor you're referring to is Boss Dharamsee, whose story is told in Chapter 35. He arrived in Shirdi with a 'clinical interest' and curiosity about miracles. Before his direct experience with Baba, he asked another devotee, Babasaheb Tarkhad, why he came to Shirdi. Tarkhad's simple reply was 'For Darshan,' adding that whatever longing one has, that desire is fulfilled. Later, after Baba performed the miracle of turning seeded grapes into seedless ones for Dharamsee, Dharamsee asked Tarkhad about the grapes he received. Tarkhad confirmed his grapes still had seeds. This confirmation was crucial, as it proved to Dharamsee that the miracle was personal and a direct response to his own inner state.