Friend Disconnecting when i join the game

Recently, i have set up a server to play on. But when i join my friend disconnects and his minecraft freezes. If he is in the game by himself on my server without me in it he is perfectly fine and is able to play. But when i join he disconects and freezes. This is also the same on other servers we have joined with the same problem and concept. Please help i have no idea why this is happening!