Friend lost all inventory on shared world but still has 36 for xp level.

I have been playing a world for months with my friend without incident.  Today when he logged in his inventory was totally gone and he was back to original game spawn like he died.  The strange thing is he is level 36 still and his spawn point was set in a bed that is still there.  Cheats are turned off this is our world to try for achievements.    I backup a copy of the game every few days and all the saves were exactly the same on them all.  Please help!