It's fixable! It will take daily practice, but if they consistently work on it they will improve. If you've already tried penmanship books, you should probably consider a tutor or signing them up for a course. There are 3 major components to good handwriting: 1: how the pen/pencil is held, 2: posture/body position, and 3: character formation (how they write a letter or number). A book can give general instructions on 1 and 2, and very good instructions on 3. A tutor / class will be able to trouble shoot incase 1 and 2 aren't being done properly and can help to break bad habits for all 3.