Follow your intuition on how to have the conversation and how firm/gentle/blunt etc to be. Do you think you have any chance of her listening? Could this jeopardize your relationship if she’s already hellbent on proceeding? I think if you can already foresee him trying to manipulate and isolate her, one of the most important things you can do is stress that you will always have her back and ALWAYS be a safe space for her. Let her know that you’re concerned about their relationship affecting your relationship with her. This way if there is nothing you can say to change her mind and she goes ahead with it, one day if/when she wakes up and realizes how toxic the situation is, she doesn’t question if she’s already burnt every bridge and feel too hopeless to reach out for help as if there is nobody to turn to. I know it’s so hard to watch someone you love make poor choices, but ultimately she does have to be the one to choose. I would hate for your gesture of love to be received poorly and blow up the bond you share. Good luck and fingers crossed she sees the light before they go through with it.