Not much to add other than yikes. Since no one knows when the connection was made, I suspect you are liable for the work on your property. It would be nice if the neighbor helped but they are facing serious bills as well it seems.

I agree pretty hard to know who did what years ago but IMO this should be a shared cost since that was already a problem from before and their crew created this big issue. Talk nicely and see how far that goes . We had a 1905 house that sat on a shared piece of property and no end of fun to get it split into 2 pieces. In the old days there was noy so much stuff to deal with I guess .

We do not share property, but it’s like we do as we are all crammed in here together. It boggles my mind that the person living here was ok with this.

Maybe I am missing something but it appears that you have a significant bargaining tool since it seems as though you could block the flow at your property line unless there is something in your chain of title that gives your neighbor an easement for the use of your lines.

Obviously I wouldn't unilaterally cut off the lines but I would check your deed and chain of title for easements and then let your neighbor know that obviously he needs to find a new drainage system and see what kind of negotiation you wind up with in terms of cost sharing.

I don’t see it that way, I’m not sure if the drain was doing anything other than draining crap into our sewerage line, or until it was dra8nibg crap into it, until and unless their backyard is completely torn up we don’t know precisely what it was doing and how. But now we have drains running at a fraction of their capacity with no easy remedy. WE are the ones who are Screwed.