How to explain that its wrong to ur ex's bff?

My ex boyfriend that I broke up with a week ago after two months of dating, my friends told him that my BFF likes him (even though she hates him) and she can only tell him that she cant date her BFF's ex. Everyone is telling him its wrond but he thinks it doesn't matter how can we explain to him that you can't date your ex's BBF mainly after a week