This isn't about you or your friend; it's much bigger than that.
If your friend is that weak of character it would be something else: alcohol, drugs, gambling...
Adults are supposed to make adult choices.
Maybe so maybe no. Nevertheless, porn is the thing that is ruining the poster's friends life. Why bring up OTHER stuff? What if the friend was a drunk instead of a porn addict? Then we would say, "Alcohol can ruin your life." And that would be true too.