AITAH for being hesitant to cut off a long time close friend?

Hello all, I (21M) recently got into a semi-big fight with my gf (21F) and wanted to see where some unbiased parties stand. This post will probably be kinda long so be warned.

It all started about 3 or 4 years ago when one of my bestfriends, we’ll call him Zack, started dating a new girl, we’ll call her Jazz. Now me and Zack had been really good friends already for about 4 years at that point, and it just so happened that Jazz was one of my girlfriend’s best friends. (Pretty irrelevant, but so that you have all the info, me and my girlfriend had just started dating at this time). So we all thought it would be fun since we were all pretty close we could do double dates and hang out as the coupled up friends in the friend group. And it was good for some time. Now to the problem, about a year ago I found out, through my girlfriend who had been told by Jazz, that Jazz had caught Zack on a dating app. Which I knew nothing about, Zack never really talked to me about how things were going with Jazz and I never really asked. Anyway, somehow Zack convinced Jazz that he was only on there to meet new people and make friends which she decided to believe and let it go. A couple months go by and I find out, again through my gf who is being told all this through Jazz, that Zack wanted to break up with her. Now here’s where shit really hits the fan. From what I’m told his specific wording was, “What do I have to do to make you break up with me?” to which she said “Idk, cheat on my ig?“ and he said “I did, I cheated on you a year ago.” Now I’m not sure if that’s what he actually said but I do know that he confessed to cheating on Jazz and he wanted to break up with her. Again he never brought this up to me at all, but we both knew that our girlfriends were bestfriends and that I would likely know about it. And if you’re thinking why didn’t I bring it up, well because it just never felt like my business to go grilling him if he didn’t bring it up himself. Now, after that Jazz starts leaning on my girlfriend for support which I encouraged and tried to do whatever I could, but then Jazz started talking to Zack again. My girlfriend would tell her to block him and go no contact, but for some reason Jazz just wouldn’t listen and kept talking to him and they would be on and off, all the while Zack was now openly talking to other girls and doing whatever. This is the point where my girlfriend starts to say that it looks bad on my part to still be friends with him because of the things that he did. I understand where she’s coming from, but I really don’t know what to do. It got to the point where I had a long talk with Zack and basically told him to leave Jazz alone and even if he feels like she was the one for him and that now that he’s been with other girls he realized she’s the one he wants to be with that that isn’t how things work. I told him that he shouldn’t feel like he could drag her along and get to be with her when he felt like it. But in the end he didn’t listen to me and now they are back together (which honestly I have no idea how Jazz feels comfortable doing that, but hey), and my girlfriend is extremely adamant that I should not keep being friends with him.

Sorry for the rant, but it’s just a lot of information and wayyy too much other stuff happened in between, but let me know what y’all think. TLDR: My bestfriend cheated on my girlfriends bestfriend and now my girlfriend says that me still being friends with him, puts me in a bad light.