Theory:

     Nishad had obviously not been paying too much attention to my list of facts. All he could say was, “Poor man, Maya, he must be so lonely if he doesn’t have any friends.”

     “How can a crook have friends, idiot?” I almost shouted.

     “At least he has one friend, the one who meets him on Sundays,” said Nishad.

     A brilliant thought occurred to me just then. “That man must be Mr Nath’s accomplice in crime,” I said. “Maybe he keeps all the loot and he comes now and then to give part of it to his partner, Mr Nath, for expenses. That’s it! I’m sure I’m right.”
Explanation:
 
Maya's list was complete, but she noticed that her brother was paying little attention to her and continued to refer to Mr Nath as a poor and lonely guy. Maya became enraged and almost yelled at him. She inquired as to how a crook could have friends. Through the rhetorical question, Maya was trying to explain how criminals do not make friends and, as a result, they are more likely to be lonely.
 
Nishad ignored her once more and remarked that Mr Nath at least had that one friend who visited him on Sundays.
 
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'At least he has one friend', said Nishad
 
Meanwhile, hearing Nishad talk about the visitor, Maya's thoughts took an interesting turn. She had "a brilliant idea" in which she was certain that the visitor was one of their suspect's accomplices.
 
She reasoned that the accomplice might have retained all of the stolen items and gave a part of it to Mr Nath whenever he visited him.
 
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Was the visitor an accomplice?
 
Important!
The words accomplice, partner, helper, and companion have similar meanings, yet they are used in different situations. Check out the meaning given below for a better understanding.
  
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
LootStolen money or valuables
2
AccompliceA person who helps another commit a crime
3
PartnerEither of a pair of people engaged together in the same activity
4
Helper       
A person who helps someone else, especially a relatively unskilled worker assisting a skilled worker, usually by manual labour
5
Companion       A person you spend a lot of time with often because you are friends or because you are travelling together
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.