I was flying back from OSH with someone on Friday, and the airport we were going into was using the localizer back course. I showed him the chart, said min altitude was 1580 feet, and at .6 DME we go missed. He was fussing with the autopilot, which had long since given up the ghost, and I pointed out to him that the CDI was set up 'wrong', not for back course, and then we descended to 1200 feet, where I said, 'hey, you're to low'. Should I file a NASA form, or it's just him that messed up. I was PNF, part 91.
Which brings up another subject, under 91, if I'm just riding in the front seat, and the PIC screws up, am I subject to sanctions, too.