[dddd]

From my off-roading experience, even the tiniest of twigs can derail the chain off of one of the pulleys, leading to carnage.

Even a small piece of paper or plastic wrap floating in the breeze, or a small stone bouncing up from the roadway, could similarly inflict mayhen on the chain's intended running path.

I don't see where the renter has cause to say that the rider was necessarily at fault here, rather it's the crap-shoot of riding in the real world that seems probably to blame.

I have seen loose limit screws and pulley bolts derail chains though, which I would say is traceable to some lack of proper bike setup.

Without the derailer here to do post-mortem on, the cause seemingly can't be determined, but with the cable housing looking the way it does, possibly it snagged violently on something, especially as the chain seems to have been forced beyond low gear. Hopefully the frame is ok.

Lastly, it might be worthwhile to read the rental contract(?).