Why was Ravi worried about what Grandfather had at the tea stall?
Ravi was worried because Grandfather was consuming items that were forbidden for him at home. At the tea stall, Grandpa sipped a sugary cup of tea, ate two bananas, and then tucked into an ice cream.
Ravi knew that his mother, who had left him in charge of Grandpa, would be extremely upset by this. He was so concerned that he thought his mother would faint if she saw what Grandfather was eating.
Explanation
The context explicitly states that Grandpa was 'forbidden sugar at home' and that bananas were 'another banned item'. Ravi's worry stems from the fact that his mother left him in charge, and she would be horrified to see Grandpa breaking his dietary restrictions, as indicated by Ravi's thought that 'Mother would faint if she saw this!'.