Hi
Firstly, before I get abused I would like to agree with all of the comments above. It has not been done correctly and it is very unlikey that there is a fault with the adhesive. If you can find out the manufacturer of the adhesive I am sure that a sample can be tested to see if it is at fault (I know this can be done with plaster, never needed to try it with tile adhesive). If they can confirm the adhesive is not at fault it will work in favour if you intend to pursue it.
Here is the unpopular bit, having looked closely at the picture I suspect the reason it has failed is down to dust on the back of the tile causing it not to bond and not the fact that it has been dabbed. I think you can see the white dust stuck in the adhesive.
The fact that it has been dot and dabbed does not necessarily mean the rest of it will fail (although no one can guarantee that). I have taken many many tiles up over the years thats have been dot and dabbed and they showed no sign of failing. The only way those tiles were coming off was with a hammer and chisel.
Good luck getting it resolved.
There is an argument to be had in saying that if you dot enough, then dab a lot, you might get the coverage percentage right and hold on.
But then there are other rules it doesn't conform to.
And one main one being as it needs 90 - 100% coverage in wet areas, and 100% no arguing for floors, then it just takes too much time dotting and dabbing.
Often done decades ago. But it's the sign of an untrained and unskilled tiler these days with the adhesives and grouts we have as options, and the tile weights that need to be installed etc.
So the neighbour needs a clip around the ear. I bet he got a nice £400 drink out of it by the sounds of it too.