Technically it is illegal to do so. He is paying for the connection per household. Inside his house, he is allowed to split as much as he wants. Since you are not the same house, you are not suppose to splice off his connection. It is no different than two seperate homes sharing cable lines or telephone lines. The difference is that unlike cable, there is no way for them to detect the extra line splice between homes. So the real question is: knowing you are not going to be able to get caught (unless you invite the able guy into your house), will you still do it?
P.S. as a side note, digging a small trench, laying down PVC, and feeding the cat5e into it is the best way to do it. You can also attach the PVC from roof top to roof top and run from attics, but it is more noticeable. Just be sure not to excedd the maxium for the switch.