This applies to Windows 10, Xbox 1, PS4, (and probably pocket edition)
Basically my friend was flying in the Nether with Elytra and he was disconnected from the game. When he spawned back in after joining back, he found himself under lava and had no way of escaping. He lost everything he had, including his mending diamond armor, which isn't completely fair as he had no time to react or escape.
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I believe Nicholas is correct.
Based on your description this is a duplicate of MCPE-42310 which speaks to the world and player loading and environmental factors affecting them before the player is given control of the character (or leaves the loading screen).
I'll close this off as a duplicate. All the best in reforging your friends gear 🙂
Ionic
Okay, here's my prediction as to what happened if the disconnect wasn't manual:
He was flying with elytra, as normal.
He lost his connection mid-flight, and it took a few seconds for his game (we'll say client-side just to keep things simple) to figure out that it wouldn't be able to reconnect, or it did but your side had already timed out.
While the world on your side (let's call this server-side, as it was being hosted from your own client like a server) was waiting for the connection on the other side so it could resync up to his client-side position, inputs, movement speed, etc, it kept whatever inputs and direction your friend had held down.
Your friend, server-side, falls into lava.
Server-side, it realizes that the client won't reconnect, so it stops simulating your friend's side and prints the disconnect message.
Since your friend has disconnected client-side, the inputs and location he made are no longer stored in their device's RAM, so when it reconnected it stuck with whatever the server had.
This should be the cause of what happened. Not necessarily a glitch, though it could be considered somewhat of an oversight.