The subject of A Long Walk to Water is still alive. He told his story to Linda Sue Park. You'd need input and permission. Without that, you risk being accused of libel. It's not just the person involved, but the peripheral characters around that person, who are also alive or have family members to act in their steads on behalf of their estates. Please consider asking in the SCBWI Ask A Lawyer portion of the boards for the real legal implications as I'm just guessing.