How to fix / replace outward facing corner of boxed in pipes? ?

In my bathroom there is an outward facing boxed in pipes. When removing the wall paper from it, it became apparent that a section is badly damaged. It has split from the wall and is rather crumbly. I belive it has been made of plasterboard or jip-rock.
How would I go about fixing / replacing this section? It is about 10 inches in height, and measures just 3 inches from each wall to the corner of the outward facing corner.