Crash when joining server

When my friend Joins my server his minecraft crashes. He can fix it once by deleting .minecraft and redownloading but as soon as he disconnects again it wont let him join.

This message comes up in the server GUI.

TextComponent{text='Disconnected', siblings=[], style=Style

{hasParent=false, color=null, bold=null, italic=null, underlined=null, obfuscated=null, clickEvent=null, hoverEvent=null}

}