Boyfriend is a very very bad person

To start I'm a guy but my sis wanted me to post this here.

My sister (19) has been dating this guy (21) for a year now, but today during supper she saw a notification on her bfs phone from another girl who I will leave unnamed but she had hearts next to her name so my sis was suspicious. She looked into who it was and it turned out to be a 16 year old school volleyball player, who my sister's bf was cheating on her with. So not only was he a liar and a cheater, but he's also a pedophile. I knew this guy was a bad person but my sis is crushed and doesn't know what to do. She wants advice from strangers because she doesn't trust me to help. Can one of y'all help her?

Thank you, God bless