What is it about the farm where he lives and its energy that troubles his partner in not feeling safe and unable to live there?

She is reacting both to negative energies being part of the natural history of the location, and amplified as well by the interlopers as a way of keeping those present off-balance and on the defensive. This is acting adversely on both of them when present, and this is also triggering deep inner karmic wounding from prior lives where she felt vulnerable, and her children particularly, in the face of the darkness. So it is an area of vulnerability within her still present because of past negative encounters leading to great unpleasantness. That need not happen. The problem now is one of healing the past so it does not spoil the present and project into the future as repeated warnings that cause unnecessary worry and disharmony.