If it was pressure washer then he washed the car and then got it dirty. Cleaned where the decal used to be and then got the car dirty some more.
Vinyl and lam types? Application Temps? If it's low temp when the install was done then the decal may have never really adhered to the car.
How did he say it happened this time?
So after further discussion with supplier and client, power washing in the extreme cold was the culprit. Water from the power washing got behind the graphics and froze. We have never seen it happen like that before .
Did you tell the client that they can't power wash the area around the decal? That's pretty standard stuff to tell a client after applying vehicle graphics. If you did and they didn't take your advise, I don;t think you owe them anything.