And what if it pushes in to the touch like quick sand?
That might suggest a problem that's been around for much longer than during the period of recent rainfall. New plaster or internal rendering won't deteriorate that quickly through mositure. However, if it's old plaster, then yes, it could easily turn to that kind of texture. Even though it's still on the wall, old plaster can easily be completely shot and very quickly turn to mush. Just because it's still on the wall - that's an optical illusion, it's actually stand upright, very delicately balanced against the wall
It depends how big the area is and what you want to get into. Once you've sorted the cause out, it will dry out with some assistance from a dehumidifier [and a contribution from you to your electricty bill]. Depending on how bad it is, I'd be inclined to hack all the damaged plaster off and get back to brick work, it will dry out much, much quicker. This obviously means repairing the wall afterwards. If you're handy then no problem, but if you feel it's a job you'd rather not tackle then it will require getting somebody to sort it out for you. Know anybody you can trust? It's not a big job, but the problem is, you could easily open a can of worms if there's not much integrity to the plaster [even the dry stuff]. Once you start ripping it off....the issue will be getting to a point where the wall is sound enough to stop!
Really depends what you want to get into
