The brief
The C180's aircon had stopped doing its job, blowing warm air out of the vents however it was set. He brought it in, which makes sense in this climate, in Singapore a working aircon isn't a luxury. Warm air from the vents usually means the compressor. The compressor is the pump that compresses the refrigerant so it can carry the heat out of the cabin, run off the engine belt through a clutch. When it fails, internally worn or the clutch packed up, it stops compressing properly, so there's no cooling, and a compressor that's broken up inside can leak and send debris through the rest of the system. A failed compressor doesn't come back, so it needs replacing, and properly: the system flushed and the receiver-drier renewed with it.
The diagnosis
A check confirmed it, the compressor wasn't building pressure, its internals worn, and it had let a little debris into the lines. The condenser, the lines and the evaporator checked out, but with a failed compressor the system gets flushed regardless to clear it before the new one goes on. That's a compressor replacement, with a fresh receiver-drier and a flush, not something a recharge alone touches.
The work
The refrigerant was recovered, the drive belt released, and the failed compressor unbolted and removed. The system was flushed to clear any debris, a new genuine Mercedes-spec compressor fitted along with a fresh receiver-drier and new O-rings, the belt set back up, the system pulled down to a long, deep vacuum, and recharged with the correct weight of refrigerant and the right oil charge. A check at the vents confirmed cold air at idle and no noise from the compressor.
The outcome
Cold air at the vents at idle, strong cooling, no noise from the aircon, and the charge holding. The C180 went home with the aircon working the way it should. A failed compressor only gets worse and can take the rest of the system down with it, so changing it, flushing the system and recharging properly fixed the cooling and protected the rest of the aircon.