Another thing;

Based on the few interaction I had with him once this started, and even though he avoided saying much, it became apparent to me that he’s been manipulating her. He mentioned that he knows everything she says to me and tells her which words to use because “he knows me”, that he’s been removing “my brain parasites” from her, that he knew my “intentions when it comes to her”, that he knows her better than me even though they only spoke for a few weeks and so on. He started talking and “being nice” to her when she was in a vulnerable and confused state due to our relationship issues, I’m more than sure there was an agenda at play, considering the situation, timing and the outcome.
Likewise during my last interaction with her (where she refused to give me back some of my property), she immediately got him to try and talk to me while being extremely hurtful and evasive. Everything that was going in between me and her since the conflict started, has been actually going through him and he told her what to say, eventually he even tried to talk on her behalf.

If this isn’t manipulation then I don’t know. To me it feels like she became his hostage. As a reminder, I did warn her about him being possessive when things started, and she herself said that she’s feeling it too.
In the end she disregarded my warnings, was unable to stop and things went past the point of no return.