As frustrating as this is for you, there is little more you can do. Your friend is an adult and is making her own decision. All you can do is love her and be there when she finally comes to her senses.
I worry that she keeps going back because he is the first man she had sex with. Sometimes, in situations like this, the woman keeps returning to what passes for a relationship to justify having made the decision to have sex. He didn’t “take” her virginity. They had consensual sex that she may now regret. She may be putting far too much importance on this guy having been her first.
If that’s the case, you can perhaps reassure her that the first time she has sex isn’t as important as the first time she has sex with someone who loves and cherishes her. This guy isn’t worth her time, her love, or her regrets.
I wish you well.