Interesting timing with this post.
Over the years I and my NP have been expected to pick sides when mono couples we knew split. Often each individual's faults would be grossly exaggerated, we soon learned to try to step back and stay clear.
We first met my boyfriend (the Surfer) while he was 1/2 of an ENM (not poly) couple. He left their home with no intention of leaving the relationship because his then girlfriend had issues with his daughter who was young (16), homeless with mental illness and needed a safe, welcoming home in order to get back on track.
Covid hit at the same time and their relationship wasn't strong enough to survive.
For a while we only saw the situation through her (the girlfriend's) lens. I don't want to dive deeply into the lies she told about him, or the damage she tried to inflict in a failed bid to win him back but it was all very toxic.
He's remained kind to her but wary, their relationship was lifechanging for both of them and neither seems to want to end communication although I'm not sure a lasting friendship can develop but never say never.
She and I were very close before they split up and during their fall out, but she got into a Mono relationship where her new boyfriend who once he understood we'd all been intimate was threatened, and along with covid restrictions put a divide between us that wouldn't have been there otherwise.
My NP and I have a fluctuating closeness with her, she's deeply damaged but also has a very sweet side because we're aware of how her lies are fuelled by insecurity we have learned/are learning to innoculate ourselves from the damage.
She's recently broken up with the vanilla boyfriend (well made him move out, not sure I believe it's over as he's listed as being in her boyfriend on Facebook and she's sloppy that way).
Anyway if the Surfer wasn't so clearly set on finding peace with her I'm not sure I'd be reconnecting but their relationship aside she, I and my NP had a genuine friendship. She knows I'm intimate with the Surfer but she doesn't know details, it clearly triggers her - and sparks her telling me lies about what he's said to her (not knowing we've gone from hooking up occasionally to texting/talking daily and discussing anything/everything).
Anyway back to the original question the Surfer told me about the devastating damage she'd done to his family while they were still together but only to ask for help navigating the situation. He asked me to keep it a secret so I have.
I told him an awful lie she'd been spreading during their breakup about 8 months into our relationship becoming official after he brought up that a mutual friend of theirs seemed to hate him. I thought it would start WW3 which is why I hadn't spoken up before but he wasn't angry or bitter, I think it answered questions.
I only handled something that happened last week by viewing her as a metamour although she's his ex, as the Surfer wants to stay connected with her. It involved targeted lies. Although I was initially really upset by her lies (knowing they were untrue) it's actually caused he and I to communicate more consciously and really cemented for me how much I do trust him, and for him to work on making sure I feel safe.
When she messages me, she often includes lies and digs about the Surfer but in person she hasn't. If she puts a rift between him and I and keeping the peace or a connection with her is more important to him than what we have I will be ok with walking away, we're on borrowed time anyway but we're finally finding our groove.
I went from being really concerned to working out that I'd treat anything she says about him that sounds wrong as a lie, and check back in with him later. Basically defusing the power of her lies, she's just playing a mono game of trying to claim territory/power.
My NP wants he and I, even possibly the Surfer, to have an intervention and explain to her we all see through her lies, but want to stay connected and mutually supportive.
None of us are motivated to tell her about the extent of deepening of the Surfers and I connection but we all want her to understand that we see eachother a lot and are extremely close friends.
Maybe she could handle the truth but I'm the one of the 3 of us, copping the lies and I'd cop any anger.
I'm not sure the intervention would actually fix anything, I obviously would prefer everything was cleaner, and life was easier when the lies felt like the truth.
She's someone who cheats on partners, I practice radical honesty with my partners... I think she finds a misplaced sense of safety by manipulating people's perspective.