And he new how to play already?
So I'm assuming he has his own account?
In which case why didn't he use it?
Also why would he want to get a feel? Does he already have an account but didn't use it just to see what the server was like and preserve his logon info from you?
Just some questions

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Not accusing you, but Staff cannot be certain that it was your friend 100% unless you have some sort of evidence. In this situation I cannot think of a solution and I'll let staff handle this. But next time, like Necahara said, I think its best if you just let your friend play singleplayer to prevent this from happening again.
Best of Luck,
~Dragon