F900 FO RPTS THE CAPT FAILED TO FOLLOW A TCAS CLB RA RECEIVED WHILE HE WAS ABSENT FROM THE COCKPIT FOR PHYSIOLOGICAL REASONS. UPON RETURN TO THE FLT DECK; HE NOTED THE ACFT IN A DSCNT TOWARDS TFC WHILE TCAS WAS COMMANDING A CLB.

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I WAS AWAY FROM MY STATION RELIEVING MYSELF AND GETTING LUNCH WHEN I HEARD 'TFC; TFC.' I RETURNED TO THE FLT DECK TO HEAR 'CLB; CLB' AND NOTICED AN ACFT MARKED ON THE TCAS 100 FT BELOW US. I IMMEDIATELY SAID 'THAT ACFT IS BELOW US' AS THE CAPT CONTINUED TO DSND. I DID NOT HAVE A HEAD SET ON NOR WAS I ABLE TO HEAR ANY OF THE CAPT/CTLR CONVERSATIONS. I WAS DISMAYED THAT THE RA WAS 'CLB; CLB' AND THAT THE CAPT WAS DOING THE OPPOSITE. I WAS ABLE TO RETURN TO MY STATION AT THIS POINT AND NOTICED THAT THE CTLR WAS RELIEVED. WE WERE THEN GIVEN A NUMBER TO CALL ZAB. WHEN WE CALLED; ATC INFORMED US THAT WE HAD 80% SEPARATION AND THAT HE HAD THE OPTION TO FORGO ACTION AND THAT HE WAS GOING TO FORGO ACTION. HE THEN RECOUNTED THE INCIDENT AND SAID THAT THERE WAS OBVIOUS CONFUSION ON BOTH THE CAPT'S PART AND THE CTLR AND THAT THE CAPT HAD MISTAKEN ANOTHER ACFT'S INSTRUCTIONS FOR HIS OWN. IN MY PERSONAL OPINION; THE CAPT COULD HAVE QUERIED THE INSTRUCTION FIRST AND THEN THE MOST IMPORTANT THING TO DO WHEN ONE RECEIVES AN RA IS TO OBEY IT. ON MY PAT; I COULD HAVE BEEN MORE ASSERTIVE AS TO MY DESIRE TO HAVE THE RA OBEYED.