A girl must have turned me down, but I still can't let go. Can I meet her best friend?

Depends on how good your girlfriend is. Ordinary girlfriends may help you. If a really good girlfriend rejects you, she should know that her friends don't like you and she doesn't want you to pester her. She may ignore you and advise you to put it down. But if you really like that girl, you can try anything, but it doesn't rule out that this girl will hate you more and more. Good luck.