How to deal with wet plasterboard?

[Keitai]

‘Mate who wants a quick solution. Baton and cladding?’

Batoning and cladding isn’t a ‘quick solution’ it is a cowboy way to go about it. There is clearly water ingress coming from outside so it’s either faulty guttering, outlet, pointing, flashing, window seals or a roof problem.

He knows but no one's going to do anything about it and building will be knocked down anyhow

 

[Keitai]

I agree that you need to fix the problem first. However, if that's not going to happen, here's an idea: affix a wall-mounted radiator / towel-rail / heater to that section of the wall and run it 24/7. It will dry out the damp plasterboard.
Sure, it's going to steadily rotting-away behind but at least it won't show in the room.
Be aware though that that water has to go somewhere as it dries and so also, a dehumidifier needed.
Edit: be mindful of the close-proximity of the curtain. It could overheat any heater placed nearby.

Plug in heater?

 

[cdbe]

Remove rad and skirting, get some of the least "bathroom" like t&g pvc panels, glue them on with no nails, seal the joint at the floor the best you can with silicone and refit the skirting. Should buy a bit of time.