Facebook Intolerance

Your friend's friend makes an intolerant comment on your friend's profile. It's highly offensive, almost bordering fascism from your point of view. But you are not sure what to do. It's not addressed to you, but it's in a cvasi-public space - a friend's wall. You do not know that person, but in a way it's just like being in a shop and witnessing a blatantly intolerant act. What do you? Do you comment? Or do you ignore it? Should you tell yourself it is just a private comment? Or should you rather respond to it, precisely because if a private comment in a public space remains unaddressed, it may look like everyone else endorses it?