I told 'em to change oil and filter, see what happens.
When was the last time your friend changed the oil filter? My friend's 2010 did the same thing. I pulled the sensor and the little screen and that didn't fix anything. I changed the oil filter and put fresh oil in it and everything went back to normal. When the filter goes into by pass, it restricts the flow of oil and sets off the caution light in the dash. I would change the filter out. Sounds like you have good oil pressure before it hits the filter.
Changed oil (5W-30) and filter, (Wix, but shorter than I'd prefer, and shorter than the Delco filter that was on there) dash gauge is now about 25--30 percent higher than it was. Still well-below what my gauge read, though.
Previous fully-warm-on-the-highway oil pressure was low-twenties, now it's mid-thirties. Cold and idle oil pressure is similarly higher than it was. The MIL light is out.
I'm no expert on LS engines. If this were a normal small-block, I'd take it as entirely adequate but "more would be better". My '88 starts up with the dash gauge pegged at 60, even though fully-warm at idle I've got ~13--15. But the sensor is in the same end of the engine as the oil pump, not at the opposite end. That may be a factor in the higher cold pressure.
Anyway, he's happy and I'm done with that issue. If there's future oil pressure problems, he can pull the engine for diagnosis.
I still have the old filter, I may cut it open and see what's going on inside.