Not "hacked". That would require a skilled person with the right tools and hacks to break in, not to mention that acquiring a password would require decoding md5 about 3 times or whatever it is, and plus, there is this convenient, much easier way...
...the so called "hacker" could have just guessed the password had it been something simple, like "asdfjkl;" or "Password1" etc.
Also, as @jacob has added, the "hacker" could have taken the information from his powder.pref, should he have made a mod.
To finish, I believe there could have been the very slight chance that he was fooled giving his account away, however I doubt that this was the cause.
So just remember, when someone else is in someone's account, don't immediately assume that it's hacking. If it were, why didn't the hacker go on to taking over several other accounts, possibly including those with moderation powers?
What happened is that his brother opened tpt, and Chemin33r was still logged in, there was no hacking or password stealing at all. Since Chemin33r doesn't log out every time, that allowed him to do that. I'm basing this off of some comments on the save, but i'm sure that's all that happened.