Soldato
Poo through the hole then block it up with Cement
Who cares what it looks like at this moment in time. Block it up and tell him not to do that again etc etc
I would care more about what it looks like if I had a brick wall and a 4 inch hole in it patched with concrete for example.
Before you go on the war path, was this standing water causing and flooding / flood risk to his house? If it were, he may have seen this as a last resort to avoid damage to his home
That's hardly the OP's problem though is it?
No matter what issues were being caused, there's no justification for passing the problem on to someone else, least of all by modifying a party wall.
I arrive home to see a hole drilled in the wall which separates the gardens of our houses. Probably about 10cm in diameter...
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Like I said, before going keyboard warrior on him try to understand what has driven him to this. A bit of a puddle on your lawn is far more favorable than a flooded house (especially since any flooding to his house would travel into yours).
what's the wall made out of? A 4" hole is not a small hole repairs may look unsightly.