My question involves juvenile law in the State of: Colorado

My daughter lives with her father in Colorado. That is where the custody order is established. Long story short, she wants to move out of her fathers house soon after she turns 18. She wants to move in to her grandmothers house (maternal) so that she can finish high school with her friends. Is this legal? Can he stop her or have her arrested as a runaway? She is terrifed to do it but she has threatened to run away before. Hopefully I did not give her false hope telling her that she can leave when she's 18. Please help.