When you see people you love walk into darkness, never stop them. Its painful not to take a stand because you see them falling. The most intelligent thing to do is step back. Honour their choice of walking into darkness. Because if you take a stand for them, they will perceive it as you are suffocating them.
And probably in their journey they are meant to fall into darkness and LEARN. No-one can change that . Give people you love only how much they are willing to receive from you. That is truly honouring of YOU because then you have more of you for your own-self.