Ugh, what a crappy situation.
No you are not a bad person. Your instincts are correct. This person is horribly bad news for your partner. She is being pulled/manipulated into some kind of abusive relationship. (I personally would consider her experience a form of rape but I also respect people's decisions about how they want to think about or label their experiences.) Someone who has repeatedly broken boundaries, and then manipulates the person afterwards is someone who has harmed and will continue to harm your partner. But you know that already.
Has she experienced abuse in the past? I ask because sometimes people who have been abused will unconsciously seek out similar people/situations - they re-enact old patterns. That 'can't say no' over and over really seems like that kind of pattern. (Please let me be clear - your partner is not at fault for his decisions/manipulations. People like that find people like your partner, who have 'fault lines' that can be exploited.)
Urge your partner to seek therapy about this pattern, this pull she feels towards this person (or maybe this type of person).
The truly horrible thing about this situation is that you cannot protect your partner unless she chooses to protect herself. And she is currently not doing that.
You can tell her what you see, a manipulative person playing on her inability to say no, someone who has already abused her and will almost certainly continue if given the chance. You can ask her to be really conscious about her choices around this person. You can lay out very clearly for her your sense of being manipulated by this person yourself. You can tell her the harm you see coming her way if she continues to see this person. You can tell her how that scenario harms you.
Think about what you can do if she stays involved with this person. Would you need to leave the relationship? You might have to, for your own sanity. (And I mean that literally.) You can set a boundary such as 'If you continue to see this person, I cannot be in a relationship with you. I cannot watch you self-destruct, watch you be harmed over and over by this person.'
That's not an ultimatum, although I see how it could easily be read as such. An ultimatum is 'You stop doing X or I will leave you.' A boundary is 'If you continue to do X, I have to leave the relationship because I am being harmed.' Do you see the difference in tone? One is a threat, the other lays out the consequences of actions taken, or not taken.
This may be a little early for thinking through this kind of thing and I hope you can avoid it. I hope she listens to you and gets away from that person. Listen to your instincts. They know what is truly going on.
