Stuck in the void when joining server

My friend is in the void whenever he joins my server now and his inventory is invisible to him. He is automatically kicked after a couple of minutes have passed. I've restarted the server, had him restart his minecraft and his computer but he is still stuck in the void whenever he joins the server. From my pov he is just stuck in place and has his armor on, but from his pov he is falling in the void and has nothing showing in his inventory either. We are running a vanilla server and he has no mods on either, he is using the correct version, and other servers/singleplayer work fine for him though.