WARNING!!!!!! CONTAINS LANDFALL SPOILERS!!!!!!!!
Morningleaf would make an awesome best friend (and she is) because she is selfless, protective, kind, brave, and loyal. She has proven that in every book and especially in book 3 Landfall, because she sacrificed herself and traveled around the south of Anok for Star (her best friend). She was even willing to go alone but Shadepebble and Brakentail insisted they go. And she was decoying herself and could easily have gotten caught and killed! She also proved that in book 1 Starfire. She took the deathblow for Star. So thats why Morningleaf is such a good friend and thats why she's Stars friend.
I agree with all these comments!