Our fence blew down in the recent high winds, my first thought was I hope this isn't going to turn into a stupid argument over who owns what, but no, the neighbour saw me looking at the fence and suggested we go halfs, no good for op but better normally if possible
if you look on the deeds the garden with the little letter T on their garden is responsible for the fence, just moved house and was told that, why get legal people invovled cheaper just repair.
As previously mentioned, no legal responsibility to maintain or even build any fence unless to fence in animals. So only a moral one, for the sake of maintaining good relations between neighbours.
As this individual has shown no inclination towards being a good neighbour at all, then the op can just ignore them & get on with their lives.