Well,the best thing would be to see if he would help,or help buy the pipe or something to help you tie into the end of that line and put it in a ditch and bury it and send it to creek,or onto next neighbors property!?[creek would be better],if you could,,,,,,thats after you take a sample and see whats coming out of line,if its anything but just plain ole water,he is in the wrong for sure,,,you would think if it was from septic system it would stink though,,,but my washer draines over a bank and dishwisher as well,,,its all well on my side of line and even on the other side of a road from nearest neighbor,,,but wouldn't think about just basicly letting it flow onto somebody elses property,,other people don't want my food particals in their yard,nor do they want bleach and suds,,so its probably just water,or you would know,,,
Will the ground [slope] allow you to run a line from end of it to a better place to drain?Even if the ground was flat,it would back up on his end,if it wouldn't run,,uphill would even solve YOUR problem,,,,,thingy