I had this situation and it turned out fine. It was my neighbor whom I know fairly well. I just told him that I was setting up my network and when the laptop was looking for my network I noticed an open one besides my own and determined it was his.

I don't know if this is applicable outside the States but just have then go to Wigle.net and they will likely be able to see their open network displayed on the Internet. That might get their attention.