I need help! My 14 year old cousin had sex with her boyfriend! Help!!?

Ok so my 14 year old cousin had sex with her boy. And then a couple of weeks later he told her he wanted to be with her forever. After a while I would spy on them and find her hand on his pants and she would rub. And then I saw her sit on him and she would start like.. Riding him only with clothes. They have been going out for a year and a week. And I don't want to ruin their relationship by telling their parents because he makes her so happy... What should I do?