Help diagnosing truck problem

[at0075]

My friend had a 05 6.0 f250. It has 123,000 miles. He's loosing coolant badly. It has me puzzled. Is intake manifold is dryer than dry. I pressure checked his coolant system and it stayed at a constant 5psi unless I hammered it up a monster hill and still never passes 8psi but those aren't even normal driving conditions. He came over and I literally put 2 gallons of gold in for him(he hasn't made the switch yet). Drove it and got the level right. It was fine the whole time. He got home and called. It was wet under the Truck but didn't come from puking. Can't find a leak to save my life. Going through that much coolant on a basis is extreme