We got up to 50 C on the ground today in the cabine on a 300 with no Apu, lovely the 14H day made it even more lovely!!!
My last captain told me there is an FAR the says you can't have passengers on an airplane above a certain temperature. I haven't found the FAR yet (I barely looked). But I think it has to be less than like 85 degrees or something in that ballpark.