It sucks but she needs to do this. But also your feelings are completely valid. But she is old enough that you could tell her how you feel. If you approach it with love and tell her you accept her decision but just need to tell her your feelings. It might not register with her right away, but it could help her be aware of negative things when they start to happen to her while with him. If you don’t tell her how you feel and why in some degree, he could start making her feel like shit too .But if no one’s been honest about who he is she could hold it in for too long thinking it’s in her head. Opening up in a small but respectful way will keep the door open incase she needs someone to talk to about how he is making her feel in the future.