Sheila had had nightly meltdowns before, we know that from Freddie Emami. We know that Freddie Emami, who was quite scared of Sheila that night, called Nevill to get him to come to London to calm her down, because nothing was working; but Nevill refused to go in the middle of the night and waited until the morning.
Probably Jeremy knew about this (significantly that his father didn´t go right away), and probably there had been other incidences.
I can see that for those reasons, Jeremy was bewildered at what to do after the call from Nevill (assuming it happened.) There was a precedent for letting Sheila calm down by herself - set by Nevill himself.
Hope yas undertand what I am saying here.