Basic Trope: The hero's friend has been brainwashed into a Face Heel Turn, possibly even becoming a Tragic Monster. The hero must now fight the friend.
Straight: The hero fights the friend and manages to convince him that he's really a good guy successfully.
Justified: The hero wants to save his friend, and can only help by fighting.
Subverted: At the moment that the hero seems defeated, the friend leans in close and explains in a whisper that the mind control didn't really work, but the Big Bad has to think it did.
Double Subverted: After the friend tells the hero he's okay, the hero drops his guard, and the friend blasts him off, calling him a fool for actually believe what he said.
Parodied: The hero doesn't want to fight the 'friend' for a petty/goofy reason.
Zig Zagged: The hero fights his friend, who is possessed by a demon. During the fight, the friend appears to go back to normal, but no one knows if the friend really is back to normal or if the demon is just acting like the hero's friend (until the hero lets his guard down).
Averted: The hero fails in redeeming his friend and ends up being defeated by the friend; alternately, the hero fails to redeem his friend and ends up killing him.
Invoked: Other friends or rivals of the hero says that only he can save his friend.