DURING NIGHT OP; CAPT OF AN LTT CUT CORNER WHILE TAXIING OFF RWY AND ALMOST HIT A TXWY LIGHT AND SUBSEQUENTLY FAILED TO FOLLOW TAXI RTE DIRECTED BY TWR TO PARKING.

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WE HAD LANDED AT IAD ON RWY 19L. THE CAPT WAS ON LOW MINIMUMS; IT WAS HIS SECOND DAY IN THE L SEAT. WE WERE CLRED TO TURN ONTO THE REVERSE HIGHSPD AND CONTACT GND; A COMMON CLRNC. AS THE CAPT TURNED HE CUT THE CORNER TOO CLOSE TO THE TAXI LIGHTS ON THE R. HE HAD EARLIER IN THE DAY BRIEFED ME THAT HE WAS NEW AND TOLD ME TO FEEL FREE TO USE THE BRAKES IF IT MEANT KEEPING HIM FROM HITTING SOMETHING; AND IN GENERAL; HE SAID 'DON'T LET ME GET VIOLATED.' WELL AS HE TURNED I SAW AND STATED HE WAS GOING TO HIT THE TAXI LIGHT WITH THE PROP AND HIS REACTION WAS TOO LATE SO I STOPPED THE PLANE ABOUT 1 FT FROM IMPACT. WE ADVISED GND WE WOULD NEED A TOW TO CONTINUE AND WE WEREN'T SURE IF WE HAD ACTUALLY CLRED THE RWY OR NOT. AS A RESULT; AT LEAST 1 JET HAD TO GAR. THE CAPT THEN DECIDED TO SHUT DOWN THE R ENG AND WHEN ROTATION STOPPED WE WERE ABLE (THROUGH HIS STEERING AND MY BRAKING) TO CLR THE LIGHT AND CONTINUE. HE WAS A BIT FLUSTERED AND MAY HAVE CONTINUED TAXIING TO THE GATE ON AN UNASSIGNED TXWY. WE WEREN'T SURE AND THE GND CTLR WAS HELPFUL AND MADE NO COMMENT ON IT. HE ALSO TAXIED PAST OUR ASSIGNED GATE AND HAD TO MAKE A 180 DEG ON THE TXWY A. THEY WERE ALSO HELPFUL AND MADE NO COMMENT ON THIS MISTAKE. WE MANAGED TO AVOID AN INCIDENT PROBABLY BECAUSE OF THE LOW PIC RULE WHICH OUR COMPANY ENFORCES WHICH REQUIRES AN SIC WITH MORE THAN 100 HRS IN TYPE TO BE PAIRED WITH A NEW CAPT.