It's complicated, but the heart wants what it wants. She seems to be moving on fairly well too, even has some new nicknames for him now that their stuff's IRL.
I'd said repeatedly that I am not built for poly relationships over jealousy issues and damages from being cheated on by past partners. I am insecure and sharing makes it feel like some sort of contest where I'd stand to lose any time an argument crops up or something, knowing the 'other' will be there as a support network that lends to usurpation sort of like we see CS doing when she shops for 'backup guys'. All of our baggage from earlier in the relationship basically sealed that from showing any allowance for it to be pure toxicity for us, lending to the one witnessing the other exploring it to get miffed even during breakups construed later to be more like 'a break'. I was assured that we were monogamous while what they were was emphasized to be 'Friends' with each allowance being over basic things, like being allowed to make friendly flirty comments as long as it doesn't go deeper than that (not being allowed that makes it harder to make friends I guess), which then expanded and hit deeper shit until it hit points that I'd guess I wouldn't be okay with... which lent to not talking as much about it if not another reason being presented that was not the insecurity I'd presumed. When she slipped to me that Inquirer as her friend couldn't date someone without her permission for example I started to get increasingly suspicious about the 'friendship', and when I'd talk about the things 'he's after' her point of view was largely 'he can think whatever he wants, what he thinks doesn't have to do with my choices'.
She tried to take me out to dinners and stuff to discuss it with me, but my unagreeable nature over something like this likely led to her trying to break up with either him or me and fail, finding the choice to be a painful one to make while preferring to not make it, finding it easier to prefer whoever she at the moment's found herself talking to, me or him. I'd even be uncomfortable with the two having eye-exchanges that showed 'more was going on', but of course I'd direct that to being about me and my jealousy and that her and I's relationship was stronger than this, as it had proven to be for years after some pretty weird stuff. My saying I was not fine with things likely increased the appetite to do it too, but saying I'm fine with it would just make for it going further that way too. In retrospect though she probably would have left me sooner if I were not a suicide risk, and this was her trying to find answers from within a situation where she for a time saw none (even with me telling her that my death would not be her fault or responsibility during some deep talks on my problems).
I'd ask about it and be given reasoning as to how it 'wasn't bad', I'd show I was uncomfortable with it and would prefer it to stop, but then it kept escalating until from the sounds of it she was talking about it in DMs apparently about 'two boyfriends' or something like that while not talking to me about it over how it wouldn't go anywhere or to protect me or something. She emphasized that novelty is one of the biggest attractions and it was overtime made to be my problem that she couldn't see other people, but even as my problem she didn't want to hurt my feelings and largely complied with that weakness of mine. The relationship had ups and downs, but milled down overtime until she hit a crossroads where she had to pick one and once it got towards the final hour she chose him.
She and I separated briefly like... thrice before patching things up each time over the seven years, and both of us made some odd choices and mistakes during those, but during one of those times I guess she got close with Inquirer and then refused to drop it. Considering in moments of my own pain I sought escape sometimes too I can't really blame her for finding refuge in him during the time she and I were separated. When the two of them were visiting in person she and I were not together anymore, but once she and I became friendly again a big part of me was ready to patch things up and try to make things 'Go Back To Normal' with me even visiting her to test what things have become. When there I saw her showing a lot of the older interest and dynamic, but she would use language out loud to remind herself that she's chosen a different path. Even when I asked if I could stay longer she seemed really uncomfortable with it, but ultimately went with it over not wanting to risk how I might hurt myself.
She'd say back then that she was fine with me pursuing other people (when it wasn't about specifics anyway, and further emphasized that once she had Inq), just how she'd say Inquirer is allowed to be with other people, but that is not something I wanted and my needs were something she agreed to adhere to as long as we were together, and in time she'd say enough to pacify me over it with strongly reduced language over the severity of the situation once it got to places I'd not be comfortable with. Before the breakup with her, with her driving away to be with her folks a bit before hitting the airport, she assuring me we would not be breaking up, that we had too much invested at this point and that she would never do that, but once I was weird over webcam about the two of them even potentially just sharing a bed she chose him instead over how my needs continued to overcomplicate things and whatever was going through her head in relation to all of this.
Unless you count emotional cheating there wasn't any, as far as I know there wasn't anything physical or whatever between them until she was single. There's likely a lot I do not know, as what began with excuses where my trust was dubious over the explanations out of jealousy in time began to become how it "wasn't my business" towards the end as more and more secrets began to form. She and I are otherwise still friends who watch shows together and talk about what's happening in our lives, plus I'm the one caretaking for the a cat that was in both of our care so she wants to see how he's doing from time to time too. I'm sure my version is missing bits and rife with bias too, but this is how I took it going with me sparing a lot of details. A lot of it was to protect me and challenge herself to 'do the right thing', but later she found going to him to be the right thing instead while seeing being around me as an emotional black hole of 'abuse' she'll keep falling into if she lets herself.
Rather than cheating I'd call it moving on, and I don't blame her for moving on when being with me kind of became a trap, when she outright admitted that returning to me would be her 'accepting failure'. Being with me probably sucks past a point and there was no real means of bridging my mono needs with her poly ones other than having one comply with the other's wishes, which for a long time considering the duration of this relationship she did. She did try, but ultimately monogamy proved to not be for her while I can't become woke enough to embrace multiple partners even with it being offered to me as a way to 'save the relationship' or whatever, and it's not like she didn't talk to me about those needs as a general thing while we were together. To her credit she tried to find ways to compromise, but there wasn't really the room to when I see mating with another as a way to lose feelings for the former, to trivialize what's shared between the two of them by spreading it thin.
We basically became best friends and roommates with the sensitivity of a couple, and her moving on was bound to happen if she wasn't otherwise ready to settle down. It was our mutual understanding of each other's weaknesses that lent to communication issues down the line. If this kept going on with him becoming an in person factor I'd probably find myself trying to hit him or something before lashing out at myself or something regrettable like that, so this is the better outcome where the most people stand to potentially find happiness by the end of it.
TLDR; To call it cheating is dubious, and she probably only stayed as long as she did over a sense of personal responsibility from knowing me to be a risk factor alongside a misplaced sense of right and wrong combatting her interest in him otherwise.
What i was getting to was it was less a poly relationship and more she dates whomever she likes while the partner is meant to be loyal. Inquirer will experience the same break as she's not capable of monogamous relationships it seems.
