Brand new timing belt broke?

I have a 2007 pontiac wave, Ie chevy aveo, daewoo, whatever.

It had 102,000km on it and hadnt had its timing belt changed yet. Something quiet deadly for these engines.
So I paid a mechanic shop to install the belt kit. Water pump, tensioner, belt etc.

700km later my car dies while driving. I have it towed back to the mechanic and he tells me the belt snapped. It was a dayco kit. I am absolutely livid. I was going maybe 20kmh when it snapped, but im assuming my valves and head are done. Maybe a piston, guess I wont know till I open it up.
Without asking he installed a new belt (assuming he did that cuz he knew it was his fault) and cranked it, before I had even been told it was the belt, and he was puzzled why it wouldnt start. I'm surprised he wouldnt know that it is an interference engine. Obviously if the valves are bent it wont run.
I just want some ideas on what he couldve done to cause it to break that quickly? Or is it possible it was just a bad belt? I saw inside the kit and the belt looked mint. Wasnt folded weird or anything. Just want some thoughts before I start pointing fingers.

Paying 800$ for $2k+ in damage is pretty sad.

Most likely the head will need to be replaced, but the pistons should be fine. Who knows if it was installation error or a manufacturer defect. I doubt the garage will own up to it if it were their fault, but they should cover everything as a warranty repair at no cost to you.