THE CAPT OF A CE680 HAD CONFLICT WITH A C180 ON APPROACH TO 61B.

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I AM A RETIRED MAJOR AIRLINE CAPT AND A RETIRED AVIATION SAFETY INSPECTOR (OPS). IN ADDITION; I HAVE FLOWN AND LOGGED OVER 12000 FLT HRS AS A PLT; FLT INSTRUCTOR AND PLT EXAMINER IN LIGHT GA ACFT. I SERVED AS PIC OF A CE680 DURING A FLT FROM ZZZ TO BOULDER CITY ARPT (61B). LAS VEGAS APCH CTL HAD CLRED US FOR A VISUAL APCH TO 61B. MY COPLT ANNOUNCED ON THE UNICOM FREQ THAT WE WERE ON A 5 MI FINAL FOR RWY 27L. HE MADE ANOTHER ANNOUNCEMENT THAT WE WERE ON A 3 MI FINAL FOR RWY 27L. THE PLT OF C180 XMITTED THAT HE WAS GOING TO TURN BASE IN FRONT OF US AND THAT WE WERE REQUIRED TO ENTER THE PATTERN. C180 TURNED AND CROSSED IN FRONT OF US WHEN WE WERE ABOUT 300 FT ABOVE THE GND ABOUT 1 MI FROM THE APCH END OF RWY 27L. WE WERE BELOW THE ALT OF C180 WHEN HE BEGAN HIS TURN AND WERE BELOW HIM WHEN HE PASSED IN FRONT OF US ON A NORTHERLY HDG. WE HAD EXPECTED THE PLT OF C180 TO CONTINUE ON HIS DOWNWIND LEG OR THE TFC PATTERN. WE WOULD NOT HAVE EXPECTED A REASONABLE AND PRUDENT PLT TO INTENTIONALLY TURN IN FRONT OF A JET ACFT ON SHORT FINAL APCH. WE WOULD HAVE BEEN HAPPY TO EXECUTE A GAR TO APPEASE THE PLT OF C180. HOWEVER; THE PLT OF C180 OPERATED HIS ACFT IN A POS THAT PREVENTED US FROM MAKING A NORMAL GAR. EXECUTING A GAR WITH C180 AHEAD AND ABOVE US WOULD HAVE CREATED A COLLISION HAZARD. WE CONTINUED OUR DSCNT AND LANDED ON RWY 27L. CUSTOMER SVC REPRESENTATIVE CAME TO OUR ACFT AND TOLD US THAT HE SAW THE CESSNA TURN IN FRONT OF US.