Plaster or paint a wall made of formwork stones?

Rhombus cladding. He pays for the material, you install the stuff. Good deal for both parties.

Talking to him probably won't bring any success, because the wall was already a contentious issue when it was built.
It was much higher and had to support his terrain.
When more and more earth kept falling onto our side, we pushed for a solution.
And that was his solution.

By the way, the construction of the wall took 2-3 years.

If anything, something has to come from us.

At least we "are allowed" to do something, because in principle it is his wall.

Would a nice graffiti make sense?
It is the east side, so no strong sunlight.

Maybe a stupid question but is one obligated to provide their neighbor with a visually appealing surface when designing a boundary, whether it's a fence, wall, hedge, or something else? If it's just a boundary construction for pure enclosure, I can understand that.

But what about if there's a slope retention of any kind on one’s own property?

Background: The neighbor dug out/lowered the natural ground level at the boundary; I want/need to raise the natural level on my side by 10-20 cm. So we are basically in the same boat regarding slope protection.

Only nothing is to be expected from the neighbor who dug out first except the statement "Go ahead and do whatever you think."

I somehow don't see why I should build something visually appealing for him if there is no financial participation or manpower involved, for me the cheapest and simplest execution applies if I don't have to look at it anyway and nothing is coming from the neighbor's side.

I would even, if necessary, leave the entire slope retention on my side; would it then be enough if I stay 5-10 cm away from the boundary instead of placing the slope retention directly on the boundary?

A graffiti is also a nice idea, but in my opinion, the main thing is that you should not put anything directly on the moisture-soaked concrete stones. Otherwise, you would first have to prepare the current surface and also waterproof it against moisture, which is a bigger effort. Besides, it will probably keep getting dirty again because of the water from the wall. Therefore, a simple substructure would make sense so that moisture/water stays behind it or can drain off. Of course, it depends on whether this is your living room view or out of sight behind the garage; I would also base my effort on that. If he lets you do something random there, that’s fine, why should he do/pay for something that he doesn’t even see? It can be done better together, but I wouldn’t want to get annoyed about it. If you don’t want to spend much money, you can also do it with ordinary, rough roof battens or boards, which you screw onto the substructure with small gaps, similar to rhombus slats, either crosswise or lengthwise; it’s not expensive and looks good in my opinion.

is one obliged to provide one’s neighbor with an aesthetically pleasing surface when designing a boundary?

.... definitely not, but if you always see it from the sofa, it’s no fun.

for me, the cheapest and simplest execution applies if I don’t have to look at it anyway and there is no complaint from the neighbor’s side.

Of course, that works, that would be enough. You are responsible for the function and not for the appearance.

I would even, if necessary, go so far as to leave all the protection on my side; is it enough then if I stay 5-10 cm away from the boundary instead of placing the slope protection directly on the boundary?

Unless you somehow come to an agreement, I would rather stay a few centimeters away. I once did it that way because the neighbor didn’t want a joint solution and later he then dumped his soil against it... Stories like these create neighborhood wars... I left it at that. But now I wouldn’t want to deliberately build something ugly either, I would find that weird too. If he then also builds his wall 3 cm away from the boundary, you have a gap of horror of 6 cm o_O. That would be really annoying for me; he should just heap his soil against it and that’s the end of the story.

.... Sure, that’s possible, that would be sufficient. You are responsible for the function, not for beauty.

And that is precisely where the author is quite mistaken. The builder of the wall is responsible for a "decent" appearance. However, for example, an unplastered concrete wall can, but does not have to, meet this requirement. I discussed this point a few months ago with a friend (civil engineer) because from my rental apartment I look onto an ugly wall of a garage. Unfortunately, in reality, it will probably come down to either a lawsuit or neighborhood feud, or tedious negotiations without success. Difficult situation. Can’t you plant anything more there (in narrow pots) to cover the wall? (Climbers/ivy, tall grasses, etc.)

and this is exactly where the writer is relatively sure to be wrong.

Really? Does something like that exist? ..... :D. In general, I always advocate for an amicable solution, which is why I would have aimed for a consensus here and done something proper on the neighbor’s part from the start. I will soon face a similar issue, and I don’t want the neighbor to feel bad when he looks at my building. But I would be interested to know where this is stipulated, if you happen to have it on hand. Additionally, there is the problem of being right versus actually getting what you want, and if things have already gotten tense, it will probably be difficult, so I would rather help myself.