It's always best if you show overall pictures so we can get a feel for what's happening. Words have a way of leaving a lot of mysteries unanswered.
In spite of not knowing, I'm speculating based on what it sounds like. It sounds like they (you, if you allow it) may be creating an erosion problem. If their new fence base is catching water and redirecting it, even though it will end up in the same place, along the way it will develop into a stream and could erode soil on your side of the wall. The stream would be forced to run along the wall because it's along the lower side of your lot. It protects the neighbor from washing, but adds to washing on your side.
I did not follow your other thread but it's likely that this project is a violation of drainage code. Why are the neighbors doing it? If it's to protect their property from runoff, they're in a sense thumbing their nose at your situation in deference to making sure they come out on top. General rules of drainage are that people cannot concentrate water flows onto someone else's property, nor can they block the natural flow of water from coming onto their property. If either of these rules were ignored, instead of the world operating smoothly, there would be colossal problems every time it rained.
Your husband wanting to let it go is his not understanding the degree to which this might come back to haunt him (unless you are the one buying and hauling soil to fill in the washed out areas when they occur ... thus it being no skin off of his nose!)
Of course, we can't be sure if it will be severe, or how severe, because we can't yet see it so as to make our educated guesses. My suggestion would be to get on top of this ASAP. If it is going to be a problem, you would not want to stand idly by and watch it being created, just to be nice ... so that you can be miserable tomorrow. If it truly is going to cause a problem, it would also be much less neighborly of you to allow it to happen, discover the reality of it and then demand that the neighbors correct and remove it, after they've put all the money and effort into it. Then there really would be hard feelings. It would be infinitely better to put an immediate stop to it. All of this depends on that little word: "if."
Show pictures in line with the wall/fence and pictures across the grade, perpendicular, looking at it.
Q
