I'm disappointed to hear that this aircraft didn't have some sort of redundant power supply for NR, people kill themselves thinking they know more than the dials. NR is very, very important
Each tach, the governor and low RPM horn are on separate circuits, either the battery or alternator can independantly supply power to the tachs, a special circuit allows the battery to supply power to the tachs even if the mastery battery switch is turned off.
So they pretty much covered all the bases, his problem was he ignored the alternator warning light despite the POH telling him to land as soon as practical and exhausted the battery it would appear. Robinson did there bit but he didn't do his !