Many years ago, two very talented players were matched. White moves his king from e1 to c1, then realizes that
checkmate is looming on the next move for black. White then states, “silly me, kings cannot move two squares”
and then attempts to move the king to a better square f1. Black naturally objected and insisted on the complete
castling move, which I upheld. Quickly the game ended.
It is interesting in how many really good players have the assumption because of their experience playing, sometimes decades, that they can step right in as TDs without reading the rule book. Quite often they are
surprised.
(Formatting corrected to remove extraneous line breaks.)
The correct ruling depends on whether the player has removed his hand from the king on c1, doesn’t it?
If, after moving Ke1-c1, he still has his hand on the king, he can still legally move the king to another square. But once he takes his hand off the king on c1 (which is what I assume happened), then I congratulate you on a correct ruling that dispensed justice swiftly and firmly, and gave each party exactly what he deserved.