What's going in her head? Why is she telling me things about her friend?

It sounds as if your jealousy is eating you up inside. Jealousy is a bad trait to have and a total turn-off. If you're jealous of some guy's (are you engaged to him or considered an item?) best friend, you best move on and find someone else to date that has no friends. Maybe she told you all of the things she did was b/c she didn't want you to have jealousy issues since there is nothing for you to be jealous about anyway.