ACFT LNDG ON WRONG RWY CAUSES ANOTHER TO GO AROUND.

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ON APCH WE WERE CLRED TO LAND ON RWY 13R WHILE 8 MI OUT ON THE VISUAL APCH. BFI USES RWY 13R FOR TURBOJET ACFT (10000 FT LENGTH) AND RWY 13L FOR LIGHT ACFT (5000 FT LENGTH). AS WE WERE ON A 1 MI FINAL WE NOTICED A LIGHT ACFT ON A L BASE FOR WHAT WE ASSUMED TO BE THE L RWY. AS THE 2 RWYS UTILIZE DIFFERENT TWR FREQS WE WERE UNABLE TO MONITOR HIS INTENTIONS. AS HE BEGAN HIS TURN TO FINAL; HE OVERSHOT THE TURN SLIGHTLY AND STRAYED ONTO OUR FINAL APCH COURSE. I MOVED SLIGHTLY TO THE R OF CTRLINE TO GIVE HIM MORE ROOM. AT THIS POINT; WE WERE BOTH ABOUT 200 FT AGL AND HE CONTINUED TO MOVE TOWARD US. I COULD MOVE NO FURTHER TO THE R SO I EXECUTED A MISSED APCH. IN A SUBSEQUENT PHONE CONVERSATION WITH THE TWR CTLR WE LEARNED THAT THE LIGHT ACFT HAD BEEN CLRED TO LAND ON RWY 13L BUT LANDED ON OUR RWY; THE WRONG RWY; RWY 13R. HE (OTHER PLT) SAID HE NEVER SAW US. THE TWR CTLR TOLD US THAT HE (OTHER PLT) HAD 'NO CLUE' ABOUT ARPT TFC FLOW; TAXI INSTRUCTIONS; ETC. WHILE ENRTE TO THIS ARPT HE HAD VIOLATED ANOTHER ARPT'S CLASS D AIRSPACE; THAT TWR WAS LOOKING TO HAVE A CONVERSATION WITH HIM AS WELL. NO ENFORCEMENT ACTION WAS TAKEN.