Beats me why they mount brake callipers behind the axle, rather than infront or even above. Sure, its a freak scenario, but if they routinely put them anywhere else than directly behind the axle, this just could not happen.
Glad he's Ok.
I note there are axle retenion lips on my other bikes, but not the Furai.
IMHO, all twin blade QR disc systems are subject to a repeated twisting force, where the fork on the disc side is deformed to a greater angle than the passive side. The movement is pretty much invisible except under extreme braking, but little and often is enough to loosen the skewer. Mine get checked every few weeks (which for the last week suggests an average of once every 4 wheel replacements!)