I have bought a Minecraft realm. I can connect perfectly every time however, my friend cannot connect. She is whitelisted, her internet is fine, she can play on other multiplayer servers but not on the realm. 

Whenever she tries to join she gets a "Connection Lost: Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host" error

I can see that she joins and then gets disconnected from the server in the game chat.