That jerk wasn't going with them to those stairs out of concern, that it might be their last farewell, but out of concern it wouldn't be (and they chicken out). He wanted to make sure they actually enter the paradise.
Also are japanese sommers really that extreme? I live in Europe, a few hundred kilometers north of the alps. Our climate is called Temperate climate (still), yet we do also occasionlly have days in summers (not every summer, but some) on which airtemperatures might rise to around 35°C-40°C midday to afternoon. There have been days, that have been even hotter. Of course these numbers don't set new worldrecords, but my point is, it does happen and it happens more than once in a lifetime. Don't those characters know hot summerdays, or why are they that impressed? In what kind of climate did they grew up? I get the idea, that the hot weather might be hard on them, if they aren't used to it, but they at least shouldn't be that suprised. Were did they live up until now? In a cave?