So there is an old dead pine tree on the property of the neighbor behind us, but for some reason, they built their fence to exclude it from their yard. We keep getting tree removal service people at our door asking us about removing it because it's inside our fence but since it lies on the neighbor's property, we tell them it's his problem.

Last night the top of the tree broke off and fell in our yard, and the neighbor is worried the rest will fall onto his house.

Who do you think is responsible here?