Submit a support ticket in General Questions with all the evidence and information you can provide. It'll be moved on to the right team and dealt with. However, you wont be told of the outcome.
To everyone being mean to the OP: Sharing accounts is, OH SO SURPRISINGLY, against the TOS. Therefor, reportable AND bannable. It's also possible the one friend is doing things against the TOS(other than account sharing) that the other is letting happen or not aware of. May as well be the jerk and get it dealt with now.
Honestly, I'd do the same thing as the OP. And, actually, I have. I'm sorry, I'm not going to let my "friends" go against the TOS without doing anything about it. That makes me just as bad as them because I'm allowing it instead of reporting it.