I would possibly ask her dad, your friend, if you and his daughter could go out for a girls day. Like maybe getting nails done etc that kinda thing. She's less likely to talk in a high tension area like her home. And try leading up to it. If her father is anything like mine don't tell him. Allow her to do that. As hard as it is, my dad belittled me about my SI so bad I resorted back to it and ended up in ER for stitiches.
If your a cutter too, let her know that. Let her know that you feel her pain etc and what she can do to get help. Offer your support as much as possible. that's the only thing I can think of.