ADJACENT VISUAL CONCERNED ABOUT FLT PATH. POTENTIAL CONFLICT.

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LNDG S AT SEATTLE INTL; WE ARE AT 6000 FT ON A R DOWNWIND 10 MI NW OF SEA. ATC ASKS IF WE SEE A GULFSTREAM AND ACR Y JET. I (AS FO) SEE ACR Y ON A R BASE FOR RWY 16R AND ASK THE CAPT IF HE SEES THE GULFSTREAM. I COULDN'T SEE THE GULFSTREAM. CAPT IS HAND FLYING (HEAD IN COCKPIT). CAPT LATER SEES ACR Y. I ASK IF HE SEES THE GULFSTREAM (AT 12:00). HE REPLIES YES. I TELL ATC WE HAVE THE GULFSTREAM AND ACR Y -- THEY CLR US FOR VISUAL RWY 16R AND THE GULFSTREAM FOR A VISUAL RWY 16L. WE DSND OUT OF 6000 FT AND BEGIN TURN TOWARD ARPT. RECOGNIZING THE GULFSTREAM ON TCASII I QUERY THE CAPT IF HE SEES THE GULFSTREAM. I STATE IT IS CLOSE IN AND LOW (ABOUT 1000 FT). WE ARE DSNDING ON TOP OF IT. (NO TA OR RA.) HE THEN SEES IT; LEVELS OFF AT 4000 FT TILL WELL CLR. THE GULFSTREAM PLTS EXPRESS CONCERN ABOUT OUR FLT PATH AFTER WE LEVEL OFF AT 4000 FT. ATC EXPLAINS THERE IS MORE THAN 1000 FT VERT SEPARATION. ON THE WALK THROUGH THE TERMINAL AND ON THE BUS RIDE TO HOTEL; THE CAPT (AGE 58 1/2) COMPLAINS ABOUT FATIGUE SEVERAL TIMES. DUTY TIME WAS 12:20. FLT TIME WAS 7:20. DAY 2 OF A 3 DAY TRIP. FACTORS I CONSIDER SIGNIFICANT: 1) HAND FLYING NEEDLESSLY (HEAD IN COCKPIT) 2) FATIGUED CAPT (AGE 58 1/2).