Rainwater drainage directly at the property boundary

Good morning,
the following matter:
We have purchased a property in the 2nd row and are currently preparing the construction road in the GFL area of the property in the 1st row. Yesterday, wooden slats that served as a privacy screen for the property to the right of the one in the 1st row had to be removed.
It turned out that behind this privacy screen, the neighbors of the property in the 1st row are simply directing all the rainwater from the garage extension completely to the property boundary, see attached pictures. Although it flows onto their property, just 1 cm next to it is now directly our construction road, or later the driveway with all the pipes underneath.
When the excavator operator saw this yesterday, he immediately said that this is not acceptable and is dangerous for the construction road and especially for later paving, as the water would wash everything away.
Of course, we want to discuss this with the neighbors, I would appreciate hearing a brief assessment here on whether this is allowed and what the simplest, most pragmatic solution might be. We naturally want to avoid going straight into a conflict.
Thank you and have a nice Sunday.

Politely point out this issue exactly to the neighbors. Legally, they are responsible for "their" rainwater.

Above all: Now, when you are building there, it is quite easy for them to find a solution. Once you have paved, it will certainly become more complicated for them as well.

I hope they are willing to listen...

Thank you for the feedback

When the civil engineer saw it yesterday, he immediately said that it wouldn't work like that and that it was dangerous for the construction road and especially for later pavements, as the water would wash everything away underneath.
Of course, we want to discuss this with the neighbors. I would be glad to hear a brief assessment here on whether this is allowed and what the easiest, most pragmatic solution could be. We naturally want to avoid going straight into a dispute.

Exactly as @lastdrot wrote, proceed accordingly.
You have received a perfect opportunity from your civil engineer. You are not the ones "complaining" to the neighbors, but you go based on the civil engineer's assessment. It's a completely different situation than if it were your idea - so make use of the civil engineer's remark.