It could be many things. Lets assume it isn't the bearing clearances, althought that is the biggest cause for low
oil pressure in high mile worn out engines.
It could be the gauge. Try a second. It could be a tired
oil pump. Many "rebuilt" engines get used
oil pumps or pumps of questionable heritage. A good standard pump will get you 35 pounds at idle hot maybe up to 50 at speed.
A high volume pump will get you 35-45 pounds hot and as much as 60 pounds at speed. High pressure pumps will get you 50-60 pounds hot and as much as 100 pounds at speed. All this is predicated on prober bearing clearances, oil viscosity, and oil temp.
I had a recently rebuilt 428 in my CC when I bought it. It had 25-35 hot and 50 pounds at speed. I had no reason to suspect the bearings but was really concerned about the 25 pounds at idle (which is really ok for an average engine).
I was planning an oil pan change anyway so while I was in there I put in a new Melling 57HV oil pump. Same engine, same oil I alway ran. The pressure went to 40-50 pounds at idle hot and 65-70 at speed.
It may not be that simple for your friend, but with 15 psi you'll want a new oil pump to go with whatever you have to do next.
I suggest installing a windage tray while you are down there. It is about $40 and can net you 10-15 horse power.
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