** What could be wrong with the air conditioner? **
Chances are, nothing at all.
** left it on all night. When he woke up, the next morning it was making a running noise with hardly any air coming out **
What usually happens when running a window air conditioner at night is that the inside OR outside air temperature drops below 70°F which causes the front evaporator (cooling) coil to frost up instead of just sweat like it should. Once totally frosted, no air movement can take place through it.
Have him remove the front grill and look at the evaporator coil. If it is completely frosted over, it will have to be turned off until it fully melts before proper air flow will return. Allowing an air conditioner to run like that can damage both the fan motor and compressor.
He can try to prevent that occurrence by turning the thermostat to one of the warmest settings. At such a setting it should allow the compressor to periodically cycle off and allow any slight frost accumulation to melt before starting to cool again. If it gets really cold outside at night, running the unit on 'fan only' is the best thing to do.
Both you and he may find the information at the following link useful?