I'm not defending the guy, but it could be an honest mistake.
I did a job in a kitchen wiring a new hob. Easy job as oven was next to it and consumer unit was on wall behind.
Explained to customer I'd remove the oven, drill through the wall run cable down behind the oven and into the cupboard next to it to connect to the hob.
Customer agreed, so off I went.
Job finished, tested and everything back in place, Customer tries to close kitchen door, which is a sliding one that hides in the wall cavity.
Yep you guessed right, door doesn't move due to 10mm cable passing through it.
Cue a very humble apology from me, a total reinstall of the cable and a phone call to a joiner friend to replace a door.
I kicked myself for not noticing the door, but also wondered why the customer didn't mention that it was there.
I measured twice before drilling, but still this occurred.
Accidents happen, its how it's dealt with that the customer remembers. Take responsibility for it, and fix it, no matter what it's going to cost you. You have insurance if its a big issue, or your working for free for a few days to cover the cost.
Give the guy a call, explain the situation and see how he responds