This sounds to me that they didn't like the tiles once it was completed and were looking for a way out. If they were that bothered about the quality of the work/chipped tiles they would have taken photos and not just gone ahead with breaking the tiles up.
A similar thing happened to a tiler I know. He'd tiled a conservatory and once completed presented the customer with his invoice. They said they didn't like the tiles now they were down (they had chosen them). They said they were going to get another tiler to break them up, remove and re-tile. He said 'I'll save him the bother' and proceeded to his van, coming back with a hammer and said he would do it himself. Funnily enough they paid him there and then.
These people are chancers and expect people to work for nothing. As all the above, either try to communicate through plumber or lastly threaten them with court.