'Heat stroke can be life threatening.'
'Wow, please place your hand on the window. Feel how hot it is already.'
'If things start to go downhill for the child, they will go downhill quickly.'
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I am not a doctor. I do not know for sure that 'heat stroke can be life threatening.' But, but, socially, that seems like the absolute thing to say, and that seems like the tactical thing to act on for the welfare of the child, and I'm kind of surprised that your husband's former boss didn't intervene (giving him some benefit of the doubt that he's a take-charge guy, etc, etc). And this seems to be where 'normal' people can play to strength, that they can say something and act on it, something which is merely good enough, without having to feel that they have to be so very sure that it's exactly, precisely true.
Maybe try for a couple of minutes, then go for the window.
But if the child starts having any trouble at all . . . Break the F___ing Window!
For Crying Out Loud!! !