As your neighbor, I would seriously give the landscape gardener a piece of my mind. Even if he himself would have preferred the board planks because they’re cheaper, the landscape gardener should have pointed out that he isn’t allowed to do that instead of happily pouring 60cm of concrete onto your property. What kind of zero is he as a landscape gardener? At the very least, he should have known that if it’s to be done with board planks, it never and absolutely not goes without your consent. And to stay friendly: who would be so stupid as to accept a concreted strip there when there are other solutions, namely the L-shaped stones.
By the way, be careful with suggestions like: drilling a bench onto the L-shaped stones or something! The L-shaped stones belong to the neighbor, and before you use a Hilti drill there, you should coordinate with the neighbor. And personally, I would probably be picky about that.
But why not plant a nice hedge in front of it? There are plenty of options that don’t grow too tall, don’t require much root space, are bee-friendly, low-maintenance, and pretty. You could also add a raised bed (but even if I don't have to drill there, I would coordinate with the neighbor beforehand).
Natural stone wall: well, I can understand if the neighbor doesn’t put up a natural stone wall there, which he himself wouldn’t even see. Of course, you can build a dry stone wall in front of the concrete side of the L-shaped stones yourself – also nice. With some houseleeks or similar rock garden plants in between, it will be an absolute eye-catcher. But probably more expensive than a hedge or perennials in front.