I found this installation the hard way while installing Hardie Board on the 1st floor matser bath. This is a cantilever section below double vanity from the master bath with all four supply lines attached directly to the underside of the subfloor. I got lucky and missed the pipe twice but hit it the third time. BTW 3 years ago I had to replumb the valve and supply line for ice maker from hot to cold in the house across the street.
At least he secured the piping. I would have used strapping to support it though. Things like this happen, even on big jobs. When i was working on the Boardwalk Hotel at Disney World, we found that someone had crossed most of the hot water lines to the tubs on the 4th floor. The worst problem was that the hot water was supplied by their boiler and a tempering valve was not installed in the domestic water supply lines. We had 200 degree water coming out of the hot water taps.
If you have never made a mistake, you haven't done much.