
I think this is highly dependent on how you feel about it. Firstly, I’m so sorry this happened to you. Secondly, she may not know that he’s that type of person but you also should prepare for potential backlash as she could “come after” you in defense of her BF. Some girls are not girls girls and would defend their man no matter what but I think if I was in her situation I would want to know 100% so I can cut all ties off with that POS but not everyone is like that unfortunately…
If you feel safe and in a good place mentally to let her know I would reach out (maybe anonymously if he knows you so they don’t retaliate?) and hopefully she breaks up with that vile excuse of a man and you can find closure. Either way, it’s important to take care of yourself first. If you are not in a good place this could mess you up more mentally if it doesn’t go well. Lastly, I want to tell you that you are strong and resilient. You deserve to find peace and I’m so sorry this happened