Your opponent was correct. The rulebook states, "In a four handed game each side shall open alternate frames, the order of play shall be determined at the commencement of each frame, and must be maintained throughout that frame. When a frame ends in a tie Snooker Rule 3k applies. The pair who play the first stroke have the choice of which player plays that stroke. THE ORDER OF PLAY MUST THEN BE MAINTAINED AS IN THE FRAME."