I think it means that when they are together, they do silly things together and let lose, thus making them become "bad" when they hang out. So I guess a friend she is so close with that they are comfortable to be "bad" with instead of restraining themselves to be polite and nice etc.
There's a bit where she said the heroine is not speaking honestly tho
I think she originally says rotten (as in an hardcore otaku) friend and that just got translated quite literally
Thats a possibility!!
I think it's their version of when we call our friends with terms like "this b*tch/b*stard" endearingly
I kinda dont understand why she's calling her bestfriend a "bad friend" is the friend a fake friend?or is it a mistranslation?
And thank goodness the heroine seems like a good person who knows that she can't mess with the prince lololol