Polypropylene pipework, that's the effect of UV in sunlight over the last 40 odd years. Only solution is to replace, you wont patch it up as whats left will be too brittle to support tape or any sort of repair. Unless you know the manufacturer it'll be hit and miss trying to get anything compatible with whats left, so you'll need to renew it back to a point where old and new can be joined. Where does it run to the other side of the wall?
It is advised to paint all exterior plastic pipework with a suitable gloss paint to protect it from the UV in sunlight.
It is advised to paint all exterior plastic pipework with a suitable gloss paint to protect it from the UV in sunlight.