Is "He was tired so he worked late" correct?

Why

Manav has the right two facts in mind — being tired, and working late — but "so" and "because" point in opposite directions, and mixing them up flips the whole meaning. "So" introduces a RESULT: "X, so Y" means X caused Y, with the cause coming first. "He was tired so he worked late" therefore says his TIREDNESS caused him to work late — an odd claim, since tiredness would usually make someone stop working, not continue. What Manav actually means is the reverse: working late caused the tiredness, which needs "because" instead, placing the cause in the second half — "He was tired because he worked late" — or "so" with the clauses swapped: "He worked late, so he was tired." Test: read "tired so worked late" literally, step by step — tiredness, therefore, working late — and notice how backwards that chain of cause and effect actually sounds once you follow it slowly. His friend is right: "so" points from cause to result, and Manav's sentence accidentally makes tiredness the CAUSE of working late, the reverse of what he meant.