I think I should tell my girlfriend she can't visit her best friend
How do you think you will benefit from keeping her from her best friend?
I think my girlfriend will stop being with me if I don't stop her from moving in with her best friend
I understand that you are worried about losing your girlfriend, but you can't tell her who she is allowed to see or not. If you keep acting that way, you are likely to drive her away.
I don't know what you mean, I just don't want her to move in with her best friend
She will likely start feeling smothered if you start trying to tell her who she can and can't visit. You'll want to reconsider this approach if you want to have a long trusting relationship with her. Otherwise she may decide that you're not worth the effort.