B.I.L.B.:Stealing best friends' girl/boyfriends

In the film, Jess loves Joe, her coach, but her new best friend, Jules, a British woman, loves Joe, too.
When they go to Germany to play an important match (they lose it), in the party, Jess kisses Joe, and Jules sees that, so they become really angry.
I’m sure that I will never steal my best friend’s girlfriend, first, because he is my best friend, and he is my best friend for something, and second, because my best friend is for all my life, and a girlfriend could be for a few days, months, or years.
We shouldn’t steal best friends’ girlsfriend, because we have to think in the feelings of our best friend.
But...if that girl is with someone that you don’t know, you are free to do what do you want

Miguel C.