The thing with porn is that once you're addicted, only a true relationship with Christ can set your friend free. They won't see the danger of it themselves until they decide that they want Christ more than they want the dark sinful compromises of this world. This is the same for any sin, any idol, in the lives of men and women.
We are called to be holy as Christ is holy and anything that taints that is sinful. Also, recall that Job made a covenant with his eyes to never look at a woman lustfully. Porn is an unhealthy habit of lust that quickly becomes a destructive bondage.
I pray that you're friend will choose Christ and be set free.