If the coolant was brown gooey stuff, he needs a new radiator. Also need to flush the block with garden hose.This is a good customer. He bought the truck online from California? As soon as he got it he took it to an accessory place and had a lift kit, wheels and tires, backup camera, nav system, new bumpers, etc. installed. Then he only had been driving it a week when he brought it to me for no heat and a couple other minor things. The cooling system was dirty so flushed it.
Not uncommon to have idiots mix red and green coolant and end up with brown Jell-O. If the top of the radiator is starting to turn brown anyway, it's time to change.
Get an OEM radiator if available. Otherwise must search the threads for the latest radiator-of-the-month-club.
Also make sure you replace the thermostat and gasket with OEM. (Aftermarket ones get spit out by these...) and make sure you orient it correctly. Then burp the system. Proper procedure is in here somewhere. Search for "burp" and you should find many.