["One of my best friend's other friends was talking bad about her to me.", "Aww that's awful.  How did you handle it?", 'I told the girl that she was wrong about what she was saying about my best friend. And then told my best friend what happened.']
Yikes.  Sorry you had to go through that.  How did your best friend take it?
One of my best friend's friend was telling me rumors about my best friend. I didn't believe them and stuck up for my best friend.