L30 controller described near separation event with two arrival aircraft when local controller failed to apply visual separation to successive arrivals as prescribed in local directives.

Narrative

I was giving OJT on the final sector during arrival push. Aircraft were reporting the airport 25+ miles away. Visual separation was being used between successive departures. The weather was not a factor; I believe ATIS 10sm and ceiling above 15;000 ft. ACR X was turned off the downwind for visual approach. ACR Y on straight in to did not see ACR X. ACR X assigned 180 to outer marker. ACR Y was assigned 190 then 170 then 160 knots cleared visual approach to 25L both aircraft inside of 10 miles from the airport with 2.9 miles in trail both aircraft talking to the tower. Tower called up and said he could not see either aircraft at which point the lead aircraft was 4.5 mile final inside the outer marker and the trailing aircraft was about 6 miles from the threshold or about 1 mile outside the outer marker. At this point he also asked me what he should do. Our local SOP states that the tower provide visual separation on successive arrivals and that they are to notify TRACON if they cannot. Also the ATIS states that simultaneous visual approaches to parallel and intersecting runways are in use. For this to work the tower has to provide visual separation between the arrivals/departures 7110.65. Both aircraft landed safely no S turns were needed for extra spacing. If tower could not see either aircraft they should have sent them around.