Why did he decide to tell the smaller aircraft to go through the final approach rather than a) instruct it to orbit or b) to extend downwind? He must have a reason but we shall never know..
Maybe by then it was too late and he thought an orbit would have caused a conflict and he was hoping to get him through the extended centreline quickly?
I don't blame the controller in this instance, he was just one of a chain of events. The fact was both aircraft were VFR and in VMC, but even had they not been, "see and avoid" still applies to IFR traffic in VMC, especially OCAS where no seperation between VFR and IFR is provided.
Perhaps landing lights on the Cessna would have helped?