You can find yourself in a tough position if you decide to keep a friend around who is doing things you don’t agree with. How do you remain a supportive presence in her life when you don’t support her choices?
Even if you are genuinely worried that things won't be okay or that he won't pull out of this, you can't show any of that. He needs you to stay calm when he is panicking. Don't you need the same?
Don't allow the emotions to lead you back down the wrong path with your ex.