Maybe she also needs some other friends. If she is only playing with this one friend at school and home (playdates), you and her teacher should encourage her to play with one or two other girls. Invite another girl to your home for playdates so that she can see that her best friend is not the only friend that she has.
Your message to her is right on target. Now, in addition to that, let her know she's entitled to feel whatever way she wants about her friend having other friends, but she may not mistreat her friend. If she does, discipline her and be sure they are doing that at schol as well. A reasonable tactic would be a ten-minute time out for any mistreatment.