This is why people shouldn't share accounts. The actions of one can affect the other's.
I think the story is really confusing. You should PM a moderator and get the exact circumstances of the ban. Right now, it seems that you are guessing or your friend may not be telling the complete story.
To contest the ban, file a ticket, but please get the story straight or else the staff may find it too hard to help you.
File A Ticket