Low oil pressure makes me nervous

My friend had a 99 GMC suburban with 350,000 miles on it and 20w50 wt kept it alive till he was going to NC to pick up a boat. His girl friend was using it at the time and had the oil changed for him while she was out running around, put in 5w30 that was on the fill cap. He had to pull over in Beckly WV and it was banging real bad, had it towed to a garage and they told him this was the end of the road.
He called me wanting to know what to do and I told him to find some kendall nitro 70w, change the oil and get as close to home as you can. He found some 60w at napa and put that in and headed home, he mad it about 250 miles from beckly to north of new philly ohio and she gave it up, 30 miles from home. Yes viscosity has a lot to do with it when you need it.