There are three kinds of mirrors commonly used in geomantic omen: flat mirror, convex mirror and concave mirror. The plane mirror can change the direction of qi energy, and the convex mirror can make qi radiate in different directions, which has the function of exorcism. concave mirror can collect qi in all directions and has the function of Naji.
Don't hang a mirror outside the gate at ordinary times, because although the mirror can block the Shaqi outside the door, it also blocks the wealth of Feng Shui. You only need to hang a mirror when there is foreign evil at your door. According to your account, there is no obvious evil outside his house. In this case, if you hang a mirror, it can only block the feng shui wealth, which may have a bad influence on your neighbors, so in this case, it will not harm people.
If he insists on not taking it down, the best way is to make a shadow wall outside your door, or put plants between the two doors, but this cannot be completely solved. Hanging a mirror can only hurt both sides. So convince your neighbors.