It's rude to date your ex's best friend.
Sally in the library at school when her best friend's ex-boyfriend walks by and says hello.
Sally wants to be friendly.
Sally says hello back to him, and adds that she's very busy before leaving the library.
Sally says hello back to him, and the two begin flirting and end up going on a date.
Sally's best friend never learns that Sally saw her ex-boyfriend in the library, so their relationship continues without conflict.
Sally's best friend is very upset and doesn't want to hang out with Sally any more.