If your friend has an option to consult with others and check the wi-fi signal in their rooms (1-4) to figure out roughly where the most significant signal drop is… Considering what you said about electrical status in that place…
Probably an AP in the spot B could help, but there might be a situation when AP in room 3 or 4 would help more — if your friend is planning to just buy it, I would just test it in those three spots to find what setup gives the best effect.
But, if there are more apartments in the building, all of them (assumption) with their own wi-fi — I would start tinkering with channels first to see if that would resolve the issue or at least boost the signal to any usable state.
If your friend has an option to consult with others and check the wi-fi signal in their rooms (1-4) to figure out roughly where the most significant signal drop is… Considering what you said about electrical status in that place…
Probably an AP in the spot B could help, but there might be a situation when AP in room 3 or 4 would help more — if your friend is planning to just buy it, I would just test it in those three spots to find what setup gives the best effect.
But, if there are more apartments in the building, all of them (assumption) with their own wi-fi — I would start tinkering with channels first to see if that would resolve the issue or at least boost the signal to any usable state.