The sum is not worth arguing with a friend about. Agree on 50/50 and that's that. You also bear some responsibility if the communication with your buddy doesn't work. As the client, you could have always intervened if something was not executed as agreed.
No, exactly not...
Where am I supposed to intervene when I look at the construction site in the evening and everything is completely covered with gravel, although this was never talked about? :D that's really nonsense now.
So the fault definitely lies 100% with the civil engineer. Now the only question is how much one should give in here...
Honestly, I tell you... friend or not, but I now have enormous additional costs for which I am really not responsible at all. I still have a pile of dirt at the back that has not been removed yet and a large part of the garden area full of gravel, which brings me nothing at all and will cause me problems with the outdoor area (planting a tree, etc.? How is that supposed to work with such a subsoil??).
And now I am supposed to say: Yes, let's just do 50/50 and that's that?