Young sister trapped in a harmful relationship and hurting herself over him

Salam alaikum. I know a sister who’s been talking to a boy from her school, and they’ve gotten into a really deep haram relationship even though they’re only 19. She can’t let go of him, despite him refusing to discuss marriage or tell his family. Her family is pretty strict but not practicing. Her dad is very tough, so there’s a good chance he’ll get furious and forbid her from leaving the house. Her brother found out she was chatting with a guy (he doesn’t know about her self-harm or crying over him). What’s the best way to handle this? Jazakum Allahu khairan.