that was new to me) as he was likely expecting the RH circuit.
Therein lies the problem. Was one runway a right hand circuit and the other a left? Otherwise he should be doing left hand turns as you say. Also did you approach at 1,500 or 2,000 ft? (Just wondering as the 2,000 ft approach falls into a similar realm to the left only rules)
It's a tough one because the other aircraft may not even have a radio so that puts aviate and navigate ahead of communicate where is should be.