Is it permissible for my father's neighbor to demolish her wall and build a sewage tank (may you be honored) near the entrance of his house, which may cause harm and leakage, especially since she wants to place her gate and yard within my father's yard, given that the property boundaries are unclear?

The preponderant opinion is that it is impermissible for a neighbor to create something on his property that harms his neighbor, and he should be prevented from doing so if he attempts it. Examples include digging a latrine next to a neighbor's wall, building a harmful bathhouse, or setting up an oven whose smoke causes annoyance. Such disputes must be brought before those in authority (rulers/governors) so they may remove the harm from the wronged party and ensure justice.