Well, of course, I’m hearing this third hand. I don’t know what the situation is between the neighbours.
However, the reason I posted the enquiry was that years ago I wanted to install wall lighting in the main bedroom.
Under no circumstances would my electrician drill into the chimney breast. I had to get a sheet of wood, have it painted the same colour as the wall & he drilled into that instead.
He explained to me that drilling would damage the chimney but it is that long ago, I can’t remember exactly what he said.
Hence my concerns about the hammering and drilling behind the person’s fireplace.