Neighbor is building a house next to the garage, neighbor's wall is wet

Update In the meantime, an expert has been here. The connection of the neighboring house to our garage is not professionally done in any respect. An overlapping sealing membrane should have been used and the silicone at this point must be replaced every two years. The entire metal construction must now be removed and redone.

Silicone must be replaced at this point every two years.

There shouldn't be silicone there either, but a polymer or whatever it's called.

Update Meanwhile, an expert has been here. The connection of the neighboring house to our garage is not professional in any respect. An overlapping sealing membrane should have been used, and the silicone at this point needs to be replaced every two years. The entire metal construction must now be removed and redone.

And who pays? What does the neighbor say?

Neighbor ordered the expert and also paid him, the repair will then be on him as well. He was certainly not pleased, but will have it done properly. We don't really have any damage in that sense, since only his wall is wet.

Well, that all sounds very good and like a little lesson in good neighborliness for the colleague. Considering that he initially wanted to indirectly blame you for the mess. Maybe it will help him to have better dealings with his fellow human beings in the future.