Sure it was your mate.. it's ok we've all done stupid things in the past j/k
Firstly, how long was the radiator running without water?
There is no need to remove the radiator water when mounting a cooler but anyway,
check all the connections and silicon hoses from the piping of the cooler.
I'll admit to my stupid moment.. I installed a front mount and piping, left the car warming up for a bit then took it for the first drive with the cooler, barely drove around the corner till I heard a huge BANG!/POP! then the car putted and stalled. I shitted myself and rolled it back into my drive way... found that I hadn't tightened one of the silicon hoses properly and it blew off. *Bows his head in shame*