The cabin manager can definitely enter the flightdeck post depressurisation.
Yeah, we know. But it is no longer 'normal' it is 'emergency' and it makes a hellava noise. And most crew are trained to sit down, where their view of the outside is non-existent.
In days of yore they could slip in, see that 'things were being done', and slip out - with saying a word or disturbing anyone. You remember that 'cabin secure flip-flap', that used to magically change to 'secure' all by itself? They cannea do that now, captn.