Possible Nexon Database Exploit?

Adding to this.
Your friend could have had a virus, or have gotten hacked himself first and thus you were indirectly added into this.

The series of events would have led to his main being hacked as well. Also note that I changed the info to my account after he had scanned his computer, before he ever logged on it again.

Edit: I've also stated that his FM mule was hacked twice before this. I failed to mention that it was nearly a month ago.