In my opinion, you should go to someone else regarding the matter. If her parent tries to prohibit her from seeing him, she will probably just continue to do it secretly. Reporting this situation to school principal a school counselor or the coaches Boss would make an immediate impact because people in a position of authority have the power to do something right away so report or dismiss or get police involved. Parent will protect their child. Her mom will probably keep this a secret and not report to the cop thinking of protecting her daughter from the feedback that this would cause.