re: Never ceases to amaze me what people will do

That was definitely intentional, but most likely because the contractor was looking for a cheap out and your neighbor has no knowledge.

I would talk to the neighbor first and if that doesn't work cap and use plenty of pvc glue.

Also, if he doesn't do anything about it I would follow-up with code enforcement. It may not be breaking any laws, but it's definitely a code violation (couldn't imagine a State/county/parish where this isn't a violation). They will not only force him to bring it back to his side, but he will have to run a drain to properly run it off his property (and not onto someone else's)