Legal Question in Criminal Law in Pennsylvania

She is a minor, she can not consent to sex with anyone more than 4 years older than her until she is 17. You don't ask her to stop seeing him, you TELL her, and you tell HIM if he touches her, she is not of the age for consent (see: www.ageofconsent.com/pennsylvania) so if he has sex with her, he will be charged and convicted of statutory rape. I'd also put it in writing, mail it to him regular mail AND Certified Return Receipt, so he can't say he did not know. I would also tell her school (in case they know him or see him hanging around the school), and I'd tell the police. Perhaps they will talk some sense into him. If he is 21, maybe he is just stupid, but there is a 50:50 chance he has been arrested for some inappropriate behavior before, so as I said, speak with the police.