[Sholar]

Show up. Say "I'm so sorry" and if she wants a hug, hug her. Then don't say much at all, really. Comments like "This is so messed up" will just upset her more, but if she is making them, you can wordlessly agree.

If you knew her brother at all, try to remember some stories about him in case she starts talking about remembrances. Be prepared for your conversation with her to veer suddenly from her crying and saying how she can't believe this is happening, to joking about random stuff from last week, to her breaking down. Just go with it.

If you had a mutual friend, visit the girl with her. Call before you go and ask whether this is a good time to go, and if not, when a good time would be.