Your friend probably knows something of the rules of logic, and is trying to apply them to that sentence.
His argument is that the sentence is equal to - The number of people is not zero, therefore there must be somebody there.
Actually, it simply means nobody is there. Zero.
It reminds me of Alice Through The Looking Glass by Lewis Carroll:
"I saw nobody on the road," said Alice to the king.
"I only wish I had such eyes," said the king. "To be able to see Nobody! And at that distance too!"