Spouse has been asked to paint a neighbour's back room. It's in a bad and
filthy condition (don't ask) but one patch of about an irregular square yard
has a damp-affected decorative surface.

It's the original applied surface from the 1930s, a low-relief 'combed'
shell effect which is on all walls and continues over the ceiling. It's not
Artex and not Anaglypta - there are no seams.

Spouse says that it feels like some kind of plastic.

Does anyone know what it could be and how the patch could effectively be
repaired to blend in with the rest of the room? The decorator the neighbour
called in originally said the only way to hide it was by plasterboard ...
not an ideal solution!

Mary