I assume "not blowing cold" implies the fan is blowing...so that means there is power to the unit.
Can he hear the compressor kick over? If not, he needs to check if there is power to the compressor. That would involve taking the top off and putting a multimeter on the wires to the compressor at various points between where the power comes into the aircon and where they go into the compressor...when the compressor tries to turn on.
Yes, check the fins both inside and outside that they are reasonably clean. Just picked up a rig this year and when I reached up inside, they were fuzzier than a stuffed animal. Just take some water and a stiff bristle brush and scrub lightly with the direction of the fins and most of it should come right off (it can be a little messy so putting a piece of plastic on the floor inside isn't a bad idea). That made a huge difference in the cold output.
After that, you are probably looking at theromostats and control boards if you haven't found the problem.
