CAPT OF A WDB EXCEEDED THE MAX SPD WHILE DSNDING THROUGH THE 10000 FT ALT LEVEL REQUIREMENT.

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I WAS THE PNF. THE CAPT IS A MGMNT; SIMULATOR INSTRUCTOR THAT WAS FLYING THE TRIP TO MAINTAIN FLYING PROFICIENCY. THE CAPT HAS BEEN ON THE AIRPLANE FOR 1 YR AND HAS APPROX 100 HRS FLOWN WITHIN THE PAST 12 MONTHS. ON OUR DSCNT TO THE ONT ARPT BTWN SETER AND PETIS (SB) ON THE SETER ARR; WE WERE CLRED FROM 14000 FT TO 6000 FT. DURING THE DSCNT I WAS TURNING AND IDENTING THE ILS RWY 26L AT ONT. OUR ASSIGNED AIRSPD WAS 310 KIAS FOR SPACING. AS I LOOKED UP FROM IDENTING THE ILS; WE WERE DSNDING THROUGH 9000 FT MSL. OUR ACTUAL AIRSPD WAS 350 KIAS; VIOLATING THE 250 KIAS BELOW 10000 FT. I IMMEDIATELY POINTED THIS OUT TO THE CAPT. THE CAPT THEN EXTENDED THE SPD BRAKES TO SLOW THE ACFT; BUT CONTINUED THE DSCNT. THIS CAUSED US TO DELAY SLOWING BELOW 250 KIAS UNTIL 7500 FT MSL. I SUGGESTED SEVERAL TIMES THAT WE SHOULD LEVEL OFF TO SLOW QUICKER. ATC MENTIONED THAT WE SHOULD 'RESUME NORMAL SPD.' FACTORS LEADING UP TO THE EVENT: MY LOSS OF SITUATIONAL AWARENESS. CAPT WAS MGMNT/SIMULATOR INSTRUCTOR AND NOT PROFICIENT -- 'BEHIND THE ACFT.' LETTING MYSELF BECOME PREOCCUPIED WITH COCKPIT TASKS (LOSS OF SITUATIONAL AWARENESS) KNOWING THAT I WAS FLYING WITH A LOW-TIME MGMNT CAPT. THIS MGMNT/SIMULATOR INSTRUCTOR HAD GIVEN ME MY LAST SIMULATOR CHKRIDE 10 MONTHS PRIOR. AND; I THOUGHT THAT HE WAS AN EXCELLENT SIMULATOR; CHK AIRMAN.