04 Loosing Antifreeze, Not mixing fluids

I have a buddy with a 5.3L 04' Silverado 4X4 Non-flexfuel with 150,000 miles on it and he has to put about a half gallon of antifreeze in every month. He drives about 2000 miles a month. Theres NO antifreeze in the oil or puddling where he parks it. He has had the cooling system pressure checked and theres "no leaks". He is the original owner supervisor for construction so it gets a lot of miles and runs most of day and does a little idling. He drives it like a grandpa and keeps it very well maintained.
Anyone had this problem?
I saw a thread where someone was talking about the waterpump gasket leaking when the truck is running ill check that the next time i see him. but any other suggestions

Might try pulling all the spark plugs and checking for evidence of coolant contamination on them. Otherwise??

prob radiator, with that many miles he could have hit something.

I have replaced a few where there was no visable sign of a leak and a pressure test showed no loss. The water pump gasket fails and when the engine is running it has a small leak and drips down the engine and burns off, only leaks while running. I have done them when they were getting close to 100,000(80,000-100,000) miles and I always drain the dexcool and replace, because you need to drain it anyways to take the pump of and replace the gaskets. It is an easy job and a cheap fix, good luck and pm me if you have any ?s Also I only use the pre mixed dex cool, as I have a well and you should only use clean pure water with no additives and I feel that the pre mix is easier then trying to make sure I get the water and mixture right.

check the plastic side tanks on the radiator for cracks especially around the lines as these are bad about cracking there.the side tanks can be replaced without replacing the whole radiator.my neighbor replaced the drivers side tank and i think it cost him $160.00 for cleaning the radiator and tank put on radiator.this was the big 8.1 radiator.