I would say a lot of things, depending on who they are.
But no judgments, none of that. I would show them that I am here for them, as a brother. I would pour out every ounce of love I have. I would pray for them to no end. I would let them know that they are loved by NOT only others around them, but by the Father of the Universe as well. That when they harm their body, my heart breaks, other loved ones’ hearts break, and most importantly God’s heart breaks. They are God’s beloved creation. They are precious, their bodies are beautiful things.
I would probably say a lot more. Of course it’s a circumstantial conversation, many things to think about before saying.
I’m praying that if you’re talking about someone you know, he/she may find peace and healing in God.