Weird Cog Issue

The other day my friend asked me to come over and take a look at his back wheel because it locked up suddenly and he fell off his bike.

Both the cog and lockring had unscrewed but there is no damage to any threads on the hub, cog or lockring. He says he didn't feel the cog slipping or anything out of the ordinary until the back wheel suddenly locked up. I put everything back together and replaced the lockring with a sturdier one but I'm at a total loss to work out how this happened, any ideas?

Sounds like a loose lockring...?

yeah, make sure he is not using a Campi or Miche lockring (unless he's running a miche or Campi hub ofcourse). The cogs seem to fit on both types but the lockrings are different. I put a miche lockring on my system EX hub on my frist fixed gear bike and it was just sitting there not holding a darn thing but still enough thread to sit there looking like it was.

I really like the miche lockring because you can use it with the Park Tool BBT7 bottom bracket tool and is way better than the hook spanners. Shame it only works with italian track hubs.

Definitely a compatible lockring - it was the original supplied with his OTP. The lockring can't have been loose because the cog wasn't slipping when he resisted the pedals - it happened suddenly.

for it to suddenly completly come off there would have been a mechanical failure, so either the ring/ cog or hub has split.

If the cog/lock ring isn't split/ torn out threads, then the only other logical and more probable answer would be the lockring came loose. If its a proper fixed hub then thats just bad spannering, if it was suicide fixed then it was only a matter of time.
I did my suicide fixed up with a lockring spanner + 3ft breaker bar, and within hours, it still came loose.