Our next door neighbour has run his soil pipe into our stench pipe that has previously been used solely for ventilation

our next door neighbour has run his soil pipe into our stench pipe that has previously been used solely for ventilation Are you saying 5that he connected his sewer outlet into your drainage system? Why would he do that and why would you let him?
There's just got to be more to this story. What is it? Back to you, Tom