My friend homewrecked a relationship and our mutuals are supporting it. AITAH?

My friend who we'll call Hollow, just got out of a toxic relationship and started rekindling with old flames. One of these flames were a guy who we'll call Pee. Pee a couple years back took care of her while she was in the hospital and during then, confessed his love for her. She was in said toxic relationship around that time, so nothing came of it and she actually ended up cutting contact all together with him. Now fast forward to the present and she has decided to reach back out to him to be friends again. It has been around two weeks since they met back up and A LOT has gone down.

When they met back up, our mutual friend and I were hanging out. We were to meet up with Hollow and pick her up to go have a night on the town and when we picked her up, Pee came out with her and Hollow introduced us plus filled us in that they were friends again and just hanging out. At her place. Pee ended up leaving after she got in the car and we went about our day. Turns out Pee has a girlfriend of almost 3 years and confessed to Hollow that their relationship wasn't doing so well and that he was having second thoughts during their hangout. Here's the plot twist though. Pee's girlfriend is one of my friends. So when I tell you I was on Hollows ass about their friendship real quick, I'm not exaggerating. She was pretty sure there was some old feelings there between them but assured my mutual friend and I that nothing would come of it. She told us she didn't want a relationship and that she put up boundaries with him because she genuinely just wanted to be friends again and to focus on herself. We told her it's whatever to be friends with him as long as they aren't crossing any lines, but that if they were thinking about doing so, Pee needed to break up with his partner and that Hollow needed to respect his relationship. I should've known by the face she made, that that wouldn't be the case.

Time hopping to the end of the day and Hollows parents told her she couldn't have us (now two of our mutuals, Pee, and I) over while we were back in town and inevitably got into a huge fight that caused her to stay at my place for the night. We packed some of her stuff and went to my place. Our mutual friends went home and it was just Hollow, Pee, and I. I was tired so I decided I wanted to go to bed. Pee and her ended up leaving so I gave them my spare key to get back inside for whenever she gets back. When I got up in the early morning they were outside my apartment still chatting and practically cuddling. I went outside and told Hollow that I wanted her to come up because it was late and so she did.

It's now the next day and apparently they confessed their feelings for each other but weren't gonna act on them and that they had a really great walk down memory lane during their outside talk. A few days go by and they keep hanging out. Then another day goes by and we all decide too hang out and go comet watching. My friend Hollow keeps calling it a triple date and of course that confuses me because Pee's in a relationship and Hollow said they didn't want one, so I thought this was just gonna be a friend group thing. Turns out Pee and Hollow were hanging out earlier that day and he decided to go over and break off his relationship because he likes Hollow and it was the final push he needed to end the relationship. We needed to leave around a certain time to catch the comets and he left to break up with his partner not even an hour before we needed to go. Not only did Pee last minute break up with his partner but then when we arrived on time. Him and Hollow were all over each other. Minutes after a breakup.

It's been about a week since that went down and I've been slightly avoiding them because it has low-key made me uncomfortable how everything went down, until today. Turns out his ex partner never knew about Pee and Hollow hanging out or how he was driving out of town to see another girl (His ex and Hollow live in the same town as I, Pee lives a few away) so not only did Pee lie to his partner during his already awful breakup but turns out he was cheating! Because not only were there hangouts secret, but it turns out his ex caught wind of news that I didn't even know (and I now know why) that they kissed a few times BEFORE the breakup. So least to say she's heartbroken and manic. Not only that but when the friend group caught wind of her telling people this, they all started attacking HER! Telling her it wasn't cheating because he broke it off the next day. She had to contact me just to tell them to stop messaging her because they wouldn't listen to her or leave her alone. I made it clear to them before all this happened that if Hollow and Pee cheated that I would call them out and tell his ex about it. Guess they knew I wasn't kidding because the whole group knew about them kissing before things were broke off and how their hangouts were secret and how Pee didn't let his ex know. I was completely in the unknown about most of it and I feel incredibly guilty for not saying anything earlier about them just even hanging out. The mutuals even invited me to a triple date with them last weekend (before everything went down today) but when I expressed how uncomfortable I was with it and how it really wasn't that appropriate, they (the mutuals) agreed with me and said it wasn't right and that Hollow wouldn't have liked it if it happened to her. But then turned around and enabled her behavior of pursuing Pee on purpose, enabled her lying to what I thought was us but just me, and enabled Pees behavior of doing stuff behind his ex's back. I'm the only one who has called them out for what they've done now that I know and now none of us are friends.. so. Am I the asshole?

Edit: I wasn't the one who told the ex, I haven't told her anything. Not really sure how she found out. I stayed out of it until today though.