Taking child on vacation [California]

My sister shares 50/50 custody of her child with the father, and she’s planning a family vacation during the school year. She wants to take her for a week, but the father is refusing, citing concerns about her missing school. It feels like he’s being petty and trying to interfere with the trip. We understand that vacations can be taken during the summer, our family prefers to book during the school year when prices are lower. Is there anything my sister can do legally in this situation?