I have a few barns, working farm, the walls seemed sound but some guttering has been leaking onto one, and the pointing was damaged. The outer layer of one wall suddenly partly collapsed recently, the wall is fine inside but the outer layer has fallen off completely leaving only rough rubble. Photo below. Can anyone tell me what if anything can be done to fix or stabilise this, to stop it going any further? Can the outer stone be put back, or does the wall need to be pulled down and rebuilt? I've had this before in another barn and patched with lime render and rubble, it's fine 10 years later but that was just a small bit, size of a window. This time it's half a wall.