Haven’t heard from my best friend since she started dating a guy 9 years older than her… HELP

Hi, first time poster! so I (22F) had a best friend, Grace (18F) hat i hung around with non stop, we actually started working together and everything was great… until she started seeing this guy she was 16 and he was 25, we said at the time it was a little odd and i was worried about her being pushed around by this older guy. who had a child he wasn’t allowed to see (he wouldn’t tell anyone why) she didn’t listen so we kept a close eye on her. over time she started becoming more and more unhappy, she just wasn’t the same joyful Grace we knew anymore. every time i asked her if she was okay she said she was fine but never said with a smile. when he was there he almost silenced her and it felt like she didn’t have a voice anymore. i ended up leaving my work and i hadn’t spoken to her since, i text her so many times but she just didn’t reply. it’s been a year now and i still think about her daily. i saw her afew month ago and said hi, she said hi with a smile then her face dropped when he came around and saw us. i am debating messaging her sister so make sure she is okay (who i use to know) but don’t want to over step. i don’t want her to hate me but at the same time ive gone so long without talking to her i’d rather risk losing her than not saying anything and finding out this weird man is trapping her in this relationship. I’m really worried this will get back to her and possible push her away. He is a known violent person, with untreated mental issues and a drug habit, this is known but we have no evidence to take to the police. do i message her sister or does anyone have any other ideas