She'd tell him if she really could. But it's too hard to explain in words he'll understand. It seems obvious. people aren't meant to live forever. It's a curse Jack is trying to get. He needs help from his insanity so she does. Anaba runs far ahead of him with everything she has. She's out of sight when she doubles back and passes him. And there's no sign of the girl.
She sits alone after she's gotten rid of him and holds the compass, watching it spin slowly around. She thinks maybe there's a trick to it. If it points to the water when he holds it but nothing when she does then there's more to it. She turns it this way and that trying to figure the thing out for a good hour before wondering if she should smash it against a rock and be done with it. In the end she just hides it. She can always come back later but Jack may be getting to her. Why not people? Why not be the two male and female people that live forever? It's too tempting.
After hiding it she goes back to find Jack. Hunting him is easy.