A friend of mine is skeptical about Sai Baba. Can you share a story about how Baba dealt with a doubter?

Certainly. The story of Boss Dharamsee, detailed in Chapter 35, is a perfect example. Dharamsee visited Shirdi with a 'clinical interest' and a skeptical mind. When Baba offered him grapes with seeds, a variety Dharamsee disliked, he internally questioned Baba's sainthood, thinking, 'If he is a saint, how does he not know That I do not like these grapes?' In response to this attitude, Baba gave him even more. However, when Dharamsee finally ate them at Baba's command, he was astonished to find they were completely seedless. This miraculous event shattered his ego, making all his 'clinical scrutiny' drop away and giving rise to a newfound love for the saint.