No doubt that unit now has very definitely knackered aircon because "the customer is always right" even though they don't know everything.
I've had perfectly working aircon that I'd been working hard to keep coaches cool ruined by other members of staff overstepping the mark and opening the windows. The coach quickly warmed up due to the influx of hot air from outside, the aircon overworked itself and spat out the coolant fluid and ceased working for weeks.
The biggest enemy to aircon units on 158s are people who don't know how to say no.
Sorry but despite this being a widely held belief it is 100% wrong.
Opening windows, doors or any other opening you can create in a 158, other rolling stock, road coach, office, fridge or any other air conditioned location will NOT break the A/C system, it just won't.
You just don't understand how A/C works if you think this. The temperature on the cold side has little effect on the unit's operation unless it gets so cold the evaporator builds up a layer of ice. The location where the blown cold air goes or the ambient temperature of that location, particularly if it is at the end of a long duct is completely divorced from the operation of the unit, it just can't have the sort of catastrophic effect on it you suggest.
Opening windows will certainly reduce or nullify the ability of a functional A/C unit to cool the inside of the coach but that is the only effect it will have. If the outside air temp is lower than the temp the A/C is set to achieve it may cause the system to switch off if the temp sensor is in the breeze from an open window, again this can lead to the coach not being cooled very well but as switching off when set temp is reached is a normal function of the unit this will not damage it in any way.
The majority of A/C units typically run at 100% all the time, most A/C compressors only have 1 speed setting (although automotive belt-driven compressors will often not be able to run at full speed when the car/bus engine is at idle), so there is no ability to "overwork" the system. The hot side (condenser) can get too hot but that again will have nothing to do with the location the cold air is vented to. The cooling fans breaking and the fans or condenser getting clogged up would be the only likely reasons that would happen.
The temp range the systems are designed to operate in greatly exceeds anything ambient British summer can generate, a unit sat all day in the direct sun will be far hotter than the outside temp both in the saloon and even more so in the roof space the A/C occupies yet a working unit will start up and run in those conditions.
The simple fact is that A/C units in moving vehicles are quite fragile, it doesn't take much to loosen a join or weld in copper pipes and the smallest crack will vent refrigerant and render the unit non-functional.
I worked with road coaches for a long time, typically you'd be doing well to get 3 years out of a new coach before the A/C would fail, only regular checks and maintenance keeps them functional long term.
