MULTIPLE RWY OP PARALLEL RWYS ACR X HAD NMAC WITH C172 ON APCH TO PARALLEL RWY. SEE AND AVOID CONCEPT.

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ACR X CLRED FOR THE ILS TO RWY 30L AT SJC. WX WAS 2000 FT BROKEN AND 7 NM VISIBILITY. AS I MADE THE 500 FT CALL TO THE CAPT; WHO WAS FLYING; I SAW A CESSNA JUST AHEAD OF US AND SLIGHTLY ABOVE US TO THE L; APPARENTLY LINED UP TO DO A TOUCH AND GO TO RWY 29; THE GENERAL AVIATION RWY. PASSING ABOUT 300 FT; HE SUDDENLY STARTED SWERVING TOWARD US. THE TWR IMMEDIATELY TOLD HIM TO GAR TO THE L. AT ABOUT 200 FT; HIS CLOSEST POINT OF APCH TO US (ABOUT 300 FT); THE CESSNA EXECUTED A HARD L CLBING TURN. I BELIEVE THIS SIT IS CAUSED BY 2 FACTORS: FIRST; THE 2 RWYS APPEAR TO BE LESS THAN 1000 FT APART. SECOND; THE CESSNA WAS CLRED TO LAND PROVIDED HE KEPT US IN SIGHT VISUALLY; WHICH HE OBVIOUSLY DID NOT. TWR SHOULD NOT LET RWY 29 TFC TURN BASE UNLESS CLR OF RWY 30L TFC. SUPPLEMENTAL INFO FROM ACN 307631. ACR X ON FINAL APCH ON RWY 30L AT SJC BELOW 200 FT. SUDDENLY I NOTICED A C172 IN A HARD L TURN AT ABOUT 45 DEGS OF BANK AT MY ALT VERY CLOSE WITHIN 300 FT. THERE IS NO WAY HE COULD HAVE SEEN US BUT THE TWR SAID THAT THE CESSNA HAD US IN SIGHT. THIS IS A VERY DANGEROUS SIT.