How do I (17F) help my girlfriend (18F) distance herself from her toxic, obsessive best friend (19F)?

I am posting this for both my and my girlfriend’s sake. We actually sat down and discussed it beforehand, and we agreed to look for advice together. We are completely on the same page about wanting to find a solution to this.

So, my girlfriend of 4 months, let's call her "Sara", has a best friend named "Chloe". The three of us actually went to the same high school but we were all in different grades. Chloe is the oldest and graduated a while ago, Sara just graduated this year, and I'm the youngest so I'm still in school.

To give you a bit more context on our dynamic: my family is really strict and I don't have much freedom. I can't easily meet up with Sara or go on trips with her, Sara and Chloe are the ones who usually travel and hang out together. Chloe is currently studying abroad and only sees Sara during the summer, but before that, they used to meet up constantly. They’ve been best friends for years, way before I even entered the picture. Because they were so close for so long, there was never really anyone around to help Sara realize just how messed up the situation actually was.

Chloe is a very lonely, borderline narcissistic person with a really unhealthy, obsessive attachment to my girlfriend. Chloe isn't overly rich, but she spends a ton of money on Sara—paying for entire hotel stays, food, and drinks whenever they go anywhere. She doesn't do this for anyone else, and honestly, she doesn't really have anyone else. Because Sara is just a genuinely kind person, I think Chloe saw her as a "savior" and used her to justify her own shitty personality. Chloe constantly tells Sara things like "you saved my life" and says she has never loved anyone as much as her. She "casually" drops lines about how she’ll never leave Sara. She even wrote a romantic fanfiction about two characters and told Sara it symbolized their relationship. I am heavily convinced Chloe has a massive crush on her, even though she's never officially confessed.

Despite claiming to love Sara so much, Chloe is incredibly toxic and emotionally abusive. She has genuinely traumatized Sara, used her past traumas against her, gets mad at her for no reason, and blames Sara for her own triggers. Chloe never shows remorse for making Sara cry, and the only time she ever shows any kind of "understanding" is if Sara spirals to the point of hurting herself. Because they've been friends for so long, Sara got completely used to the gaslighting and always blamed herself whenever Chloe made her feel like crap.

A few years ago, they went on a trip abroad together. Chloe is a minor influencer who values fame and content over everything, so she spent the whole time filming, ignoring Sara, and treating her terribly. Sara was so deeply traumatized by that trip that she actually became suicidal.

Just a few days ago, they went on another small two-day trip. Chloe started acting mean again, which triggered Sara's trauma from that awful past trip. When Sara honestly explained why she was upset, Chloe didn't comfort her at all. Instead, she attacked Sara for "being triggered too easily" and told her she should control her thoughts.

Chloe also got incredibly mad at Sara because she assumed Sara thought she hadn't changed at all since that terrible old trip. Sara was honestly just trying to talk to her and avoid a fight, but Chloe always wants to "win" every single interaction. She always wants to argue and actively gets mad when Sara doesn't argue back. In fact, because Sara stayed completely calm while Chloe was pissed, Chloe accused her of "ragebaiting" her, which only made Chloe get even madder.

Chloe said some horrible things and made her cry pretty badly. She only backed down and showed remorse when Sara had a severe anxiety attack and literally couldn't breathe. Afterward, Chloe tried to deflect by making it about her own effort, saying things like, "I've never loved someone this much before, I am trying my best," and, "I have never tried to understand someone this bad, I'm trying." When talking about Sara's trauma from their past, Chloe bluntly said, "I can't tolerate bullshit, so i wanted to fix it and be done with it." She tried justifying her behavior by saying, "I am a human being, I make mistakes. I get mad too."

Chloe even suggested they use a "safeword" for when she gets mad. But when Sara asked her, "What can I do when you get mad?" Chloe couldn't think of a single actual answer.

Sara told me they "made up" afterward and that Chloe said, "We will get through this together." But I am so certain Chloe isn't the type of person to change, and she is a massive risk to my girlfriend's mental health.

I want to emphasize how deeply I love and trust Sara, and just how incredibly strong, secure and healthy our relationship is. We actually have the kind of trust where we can sit down and talk about hard stuff like this without any judgment, we are always completely honest with each other. The only reason we’re even making this post is because we want to look out for her mental health together and figure out how to handle this mess together in the best way we can.

I am absolutely not the type of partner to force my girlfriend to stop talking to people or give her an ultimatum. I want Sara to realize the reality of the situation and choose to break off the friendship or at least put some distance there of her own free will.
But Sara is really struggling to let go. Their history isn't entirely bad—there are good memories mixed in there too. Plus, Sara feels a bit indebted to Chloe because of all the money Chloe has spent on her, and she feels incredibly guilty because Chloe genuinely has no other close relationships besides her.

How can Sara safely stop talking to Chloe, or at least slowly distance herself, without causing a massive blowout or a dangerous reaction? How can I best support her through this?

TL;DR: My (17F) girlfriend "Sara" (18F) has an obsessive, emotionally abusive best friend "Chloe" (19F) who treats her terribly, gaslights her, and gets mad when Sara won't argue back. Sara wants to distance herself but is struggling to let go because of their long history, good memories, and feeling indebted over money Chloe has spent on her. We are looking for advice on how Sara can safely pull away without causing a dangerous blowout.