1, I can't say for sure, but I think it's a free standing garage. Plus it was standing just fine without water pressure on it before he started. Plus water barely puts any pressure outward anyway (most of the pressure is right near the floor).
2, 3,000 gallons is 11,000l. So how is it going to leak 250,000l of water? I don't think any residential water meter could put out that much water in a week! And that for a full pressure leak. So totally bogus.
3, he specifically did not cut into the slab.
That much water? What much water? He's not dumping anything. And if it was a regular pool he would be doing exactly that, without you getting all worked up over it.
You want to explain problems with what he did - fine. But don't exaggerate, and make up non-existent problems.
2, It leaks and 250,000l of water flows out under the house washing away the soil that the foundations stand on. His neighbours house falls into the hole.
3, The houses were built on a concrete raft (common where the ground is unstable) he has just cut a big hole in the raft, it cracks and his nieghbours house falls over.
That's ignoring the risks from dumping that much water into the drainage system, leaking chemicals, humidity/damp etc.