What does it mean when a friend dreams of you getting stabbed in the face?

Dreams are about the dreamer. So in this friend's dream, "you" represent some aspect the the friend's own self. The face is a symbol for reputation, involving issues of honor or shame, respect or disrespect. The dream suggests that the friend feels that his/her reputation has been attacked in some way, and the feeling is so shameful that the dream substituted a different face for the dreamer's own.