Romilda Vane knows where you can put it.

( WARDED HEAVILY PRIVATE TO SELF )
Women of Hogwarts, I have heard your cries for help and I have answered. I'm putting together a sort of girls' guide to help my gender navigate the broad world of love, friendship, and fashion as peacefully as possible considering the various circumstances that may crop up in their paths. For example:

TIP #1: It is never okay to date your best friend's ex. Even if she says it's okay, it's totally never okay. In the first place, why would you want to put your tongue -- or anything else -- where your best friend's has been unless you have a lesbian crush on her or something? And, in the second place, why would you want to be with a guy who could, and might be, comparing how you are with him with how your friend was with him? That's just asking for trouble.

HELPFUL HINT: If you and your best friend have the same taste in men, try making a list of blokes you both think are sort of cool and dividing them up between you long before any crushes have a chance to develop. That way, it's less likely for the two of you to find yourself fighting over the same one and, on the off-chance you do fight, you have a better way of settling than just having a cat fight and not speaking to one another for days.