Theory:

      “Ramesh told me something very strange, Maya,” Seven added. “Almost every Sunday, he carries two lunches to Mr Nath’s room and the same man is with him each time. He’s tall, fair, stout and wears spectacles. Ramesh says his visitor talks a lot, unlike Mr Nath who hardly speaks.”

     “Well done, Nishad,” I told him. “Now that we’ve made some progress with our inquiries, we’ll have to sort out all the facts like expert detectives so that we can trap the crook.”

     “How you do go on, Maya,” Seven sighed. “How can you possibly imagine he’s a crook? He looks so ordinary!”
Explanation:
 
After explaining Mr Nath's eating habits to Maya, Nishad informed her that Ramesh had told him something odd. Mr Nath had a visitor almost every Sunday, and it was always the same man. Ramesh would then carry two meals to Mr Nath's room- one for Mr Nath and the other for the guest. The visitor was a tall, fair, fat, and spectacled man. Ramesh further added that the guest was quite chatty, unlike Mr Nath.
 
Maya was delighted with the findings as she believed that they had made some valuable progress with their investigation. Thanks to Nishad, they had learned more about Mr Nath. They knew what he ate or that he tipped well. Moreover, they also came to know about the visitor.
 
Maya then praised Nishad for his efforts and then stated that in order to get the "crook", they would have to sort through all the data like experienced detectives.
 
The word "crook" put Nishad off. He groaned and expressed his disbelief that his sister would refer to such an ordinary-looking individual as a criminal.
 
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Mr Nath's visitor was a tall, fair, stout man who wears spectacles
 
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
StoutOf a person who looks rather fat
2
InquiryAn act of asking for information
3
SighTo emit a long, deep audible breath expressing sadness, relief, tiredness, or similar
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.