SINGLE ENG CESSNA TURNS BASE FOR RWY 13L AT BFI ACROSS FLT PATH OF LJ35 ON FINAL TO RWY 13R. NEAR MISS ENSUES.

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CLRED FOR A VISUAL APCH TO RWY 13R AT BOEING FIELD. WE CONTACTED THE TWR WHILE TURNING ABOUT A 6 MI FINAL FOR RWY 13R. BFI TWR CLRED US TO LAND ON RWY 13R. AS WE APCHED THE RWY; WE HEARD THE TWR GIVING A CLRNC TO LAND TO A SINGLE ENG CESSNA ON RWY 13L. AS BEST AS I CAN RECALL; THE CESSNA WAS INSTRUCTED TO PASS BEHIND A LEARJET (WHICH WAS OUR ACFT) ON FINAL FOR RWY 13R. AT ABOUT 300 FT; WE SAW THE CESSNA IN FRONT OF US; PASSING R TO L ON A VERY CLOSE-IN R BASE TO FINAL FOR RWY 13L. I MANEUVERED BELOW NORMAL GLIDE PATH IN ORDER TO ENSURE COLLISION AVOIDANCE. WHEN OUR FLT PATHS CROSSED; I WOULD ESTIMATE THERE WAS 50 FT HORIZ AND 25 FT VERT SEPARATION. THROUGHOUT THERE WERE NO RADIO XMISSIONS OF UNUSUAL NATURE FROM ANY SOURCE. BEFORE SHUTTING DOWN ON THE RAMP; I ASKED THE CTLR FOR THE BFI TWR LANDLINE PHONE NUMBER. I EXPLAINED OUR OBSERVATIONS TO THE BFI TWR CTLR. HE DID NOT APPEAR TO BE AWARE OF ANY PROB OR CONFLICT UNTIL I CALLED HIM. CONTRIBUTING FACTORS: 1) CESSNA PLT EITHER NOT UNDERSTANDING OR NOT COMPLYING WITH ATC INSTRUCTIONS. 2) TWR CTLR NOT OBSERVING AND NOT VISUALLY VERIFYING ACFT COMPLIANCE.