Hi volume oil pump

If he does in fact have a electric pressure gauge it can be the sending unit as Dartcharger said. I had a 74 Monaco that showed low oil pressure on the factory dash oil pressure gauge and it was a bad oil sending unit. It was not the on/off type Ace described that is used in the "idiot" light system. It's the type used for an electric gauge which is a variable resistor. If he has a manual gauge testing it against a known good gauge is the way to go. As Terry says a HV pump with a stock oil pan is a NO-NO. I did that once and watched it suck the pan dry at 6000 rpm and before I got it shut off it spun a bearing. I was bummin to say the least.

Either way about it that's quite a drop in oil pressure and warrants an inspection.