okay, if you mean that if you ever were to cheat on your girlfriend and they were best friends, the people you are cheating on, that would be totally wrong. they would tell each other and they would...
People can (and often do) have close friendships that are not erotic. Sometimes these friendships last till one of the friends dies.
Answer
Some girls are just looking for a male to confide in. Or...
well, first you have to act a little different around her, maybe a little nicer and choose to hang out with her more instead of going with other friends. If you get the courage, just ask her out....