If he was a cowboy the tiles would be wonky. That's all there is to that.
I had a similar problem years ago when the Abergele cowboy I was working for gave up and left me twit. Since the people were lovely I stayed on and coped. I had a trial and error period which was nasty and painfull until I got to the bit that was highest out of true and then just went back and made the pieces fit.
It turned into a nice enough job after that and when they came in and admired it, that put the plate on the door as far as I was concerned.
He had told them their walls were leaking and had them spend a fortune trying to fix what was more likely a chimney tar problem and finally after I built a gate pillar that he'd also given up on, he came back in the night and wrecked it. The spitefull, little man.
I bet he is still in business. And giving customers an hard time when he realises he has bitten off more than he can wrangle his little spurs out of.
.....
Effing cheek on the OP complaining about the way he "prepared" the wall. I'd have told him to stuff it if he said anything about that. Ripped the tiles off and gone home, too.
If the man sees what the ploker wrote about him here I would bet on the plumbing giving trouble in the not too distant...
It would be a serve him rite.