Personally, 30 degrees is my limit. For an extended period 28 degrees or so, but you have to monitor the cabin yourself. Temperature monitors have been known to tell porkies. But if an airline dispatches an aircraft with an unserviceable APU in hot or cold weather to an airport without a/c at the gate they should expect the unexpected. CAT3C has the sensible solution. Failing that the aircraft is unfit for people and you wait until the ambient temperature is OK. Only common sense is required.