The unwritten rule that states that before any man can date a woman, her best friend must approve of him. If the man fails the best friend
test, the relationship will either
a) cease to exist
or
b) exist in a miserable state until said best friend finally convinces her friend to dump the man
The best friend
test is voided if said best friend steals the man, hence ending not only her
best-friendship with the woman in question but also creating a new best friend
test with her new best friend.
Confusing? Yes. But perfectly logical.