This feels a little bit similar to something my partner and I are going through. We have been having some real serious talks about where on the open/poly spectrum we are and what each style of relationship would look like for uz. It has been further complicated because he's seeing someone new who so far he has been telling me is casual but clearly they have a strong connection. We are at a bit of a crossroads in terms of if we continue on as primaries and go up the relationship esculator and buy a house and try to have a child or downgrade to secondary/part time lovers who are good friends but life takes us on our roads.
We actually sat down and wrote out all the details of the two different relationship options and made two lists with catafories which we could look at side by side. Categories included things like the level of interaction with other partners (we didn't specify emotions because you cant control that but you can control how you behave) our living situation, how often we would see each other etc etc. It took a long time and was super thorough.
One thing that I wanted to make really clear for him was that if he wanted poly and wanted to have a 50/50 emotional split between me and this new person than he was not going to get 100% of my domestic energy (chores, planning holidays, living together, generally organising our life and looking after him). If we went poly I would be gearing more towards a solo poly situation, maybe spending two or so nights with him a week. Also I would be looking at all my options as far as creating a family and he would not automatically be part of it and I would no longer use condoms with one of my other partners as both me and this guy are on the same page about condoms but have been using them out of respect for my primary partner.
I think one of the reasons this worked really well for us is we were open to both scenerios, I was fully on board and ready to deesculate the relationship if that's what he wanted. But I was adamant I would not be at home raising his baby/doing his laundry/cleaning up after him while he was giving someone else the same level of commitment as me.
So even tho he does really like this new person he decided he wanted to keep them (if they agree) as a friend with benefits and keep his primary relationship with me. We both compromised on how that would look as well. Anyway there are limits on his relationship with her, and there are limits on my relationships with others, he has also imposed some limits on himself such as not kissing her despite this not bothering me at all, but he feels he needs it to keep himself from getting to entangled with this person. Does it suck for her, yeah probably they seem really into my partner but partner and I talked at length about the nuance of wording things neutrally and not making out to be a victim. What they decide to do is their business, I trust him to uphold the boundaries we agreed too. I absolutely would not agree to talk to her about it, because she is allowed to advocate for what she needs in her relationship with him and it's up to my partner to weigh up if her wants work for him and then communicate to me any changes. From there, depending on what it is, I would probably de esculate with him and he is also clear on what level of commitment he can expect from me under different scenarios. I was very worried that she would see her and my partner as being star crossed lovers of sorts and me just an obstacle to overcome, but I do trust my partner to not tolerate them talking disrespectfully about me and outside of that I can't control what she thinks.
Anyway this was a big long ramble and I really hope I haven't made some glaring faux pas that gets me downvoted. It just struck me that you and your partner are not on the same page on being poly or open and those are two very different relationship styles and it has taken my partner and I a very long time to deconstruct all the different assumptions we had about how life was going to work.