i think a over voltage / reverse voltage protection would be fairly simple to do.
I am probably pretty naive again thinking that surely people would know to not send 12 V into a USB plug cuz nothing do that unless we are are talking QC charging in phones ASO, and even then it will only step up the voltage if the devise plugged into such a charger request QC operations.
I use a QC 3.0 mains charger myself for testing dashcams on the computer table, but the dashcams do not request QC so the charger only feed the cameras 5 volts, but as i recall the charger can do both 9 and 12 volts too is requested / supported by whatever devise plugged into the charger.
As far as i know all dashcams that are indeed 12 V, they use a barrel plug for power and not a USB plug, in that case it would be fine cutting the plug and wire into the fuse box.
The only 12 V dashcam i ever owned was my first proper 1080p system the Lukas LK-7500, but i think thinkware and blackvue ( other korean brands ) also wildly use 12 V
If the guy just cut the 12 V plug off he should be fine, but if he cut the plug and the box a little up the wire off, then there would be trouble, but i think he cut at the plug and so he are still on the 12 V side of the PSU box.
It would not make sense to cut off the 12 V plug 1 M up the wire, the sight of the square box on the wire should also make alarm bells ring in a DIY persons brain, cuz that is probably there for a reason. In this case it is the 12 - 5 volt transformer.