RPTR OBJECTS TO FLYING AS FO TO LESS COMPETENT OLDER; MORE INFIRM PLTS.

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DUE TO INADEQUATE AIRLINE STAFFING; I AM THE ONLY RESERVE PLT (CAPT OR FO) AVAILABLE IN MY BASE. I HAVE REPEATEDLY BEEN ASSIGNED TO FLY AS A FO; I HAVE FOUND AMBIGUOUS DEFINITION OF DUTIES IN SOME INSTANCES DUE TO VAGUE/CONTRADICTORY MANUAL AND CONTRACT LANGUAGE AND DUE TO TOTAL LACK OF CRM TRAINING; ALL OF WHICH PROMOTE A WIDE VARIETY OF PREFERENCES AND TECHNIQUES. ADDITIONALLY; I AM NOT USED TO; NOR DO I ENJOY; ACTING AS A SECOND IN COMMAND TO SOME OF MY COLLEAGUES. IN THESE UNCOMFORTABLE CIRCUMSTANCES; INFO 'SHARING' CAN BE DIFFICULT. IN THIS INSTANCE; THE PIC (PF) MISREAD THE DME DATA DISPLAYED ON HIS EHSI; WHICH CAUSED HIM TO BREAK OUT ON THE ILS TOO HIGH; TOO FAST; TO LAND ON THE ILS RWY (34R). I HAD QUERIED HIM; AS HAD THE TWR; AS TO WHETHER HE COULD COMPLETE A LNDG. I DID NOT REALIZE HE WAS DISORIENTED. FELT HE WAS OBSTINATELY PRESSING ON. AS IMMEDIATE SAFETY WAS NOT IN QUESTION; I TOOK NO ACTION. ON REACHING VFR CONDITIONS; THE CAPT REALIZED HIS ERROR; WAS QUITE STARTLED. WE EXECUTED A VFR GAR TO A VFR LNDG; IN THE COURSE OF WHICH THE CAPT EXCEEDED THE ALT ISSUED BY THE TWR (TO MAINTAIN AT OR BELOW 1500 MSL); AS WELL AS THE 200 KIAS ATA SPD LIMIT. I POINTED OUT THESE DEVS; HE MADE NO OBVIOUS EFFORT TO CORRECT THEM. THE TWR MADE NO COMMENT. I HAVE FOUND THIS CAPT TO HAVE POOR EYESIGHT; TO BE VERY HARD OF HEARING.