The only thing I can think of right off is maybe the neighbor (or whoever lived there before) is stealing power. Of course, the easiest way to see this is wait for the neighbor to turn on the AC, trip your friend's main, and see if the AC turns off. (assuming it's stolen after the main)
The only other thing I can think is maybe harmonics, or the neighbor has no neutral or an undersized neutral, causing one hot to go over voltage, and since it is off the same pole, might cause one of your friend's hots to go high, causing in higher current draw. Of course, I'm not too sure about this.
When I was living in my old duplex, my neighbor was stealing power from me. The basement was divided, but I saw a wire from my panel go to his side to his AC (I did the same thing, trip my main, his AC went off. I connected his 120V cable to 240V and blew his AC, then I cut it back and sealed the hole. Maybe something similar is happening here, a wire "disappears" somewhere.
