How to tell a friend she is in a very toxic/emotionally abusive relationship?

I posted earlier asking if I was the asshole for telling my friend I can't be around her and her partner anymore because they are so outwardly toxic to each other that it makes me uncomfortable. After chatting with another friend, I've realized there is a much bigger problem: she's in an emotionally abusive relationship.

My (33F) friend Rebecca (33F) has been dating her boyfriend (33M), Zach, for technically two years. Their first year was long-distance as she lived away from our hometown, where he has been living. After a year, she moved back home and directly into an apartment with him. Before she officially moved back, I raised alarms because I was concerned about her making such a big and official move with someone she didn't know very well. Even though they'd been dating for a year, they haven't spent more than a week at a time together and haven't seen each other in very many situations. I suggested it might be more wise if she moved back to our hometown but into a place of her own so they can get to know each other in a stable and realistic environment. Obviously, she did not do that.

I think it's important to note that our hometown has a very reasonable cost of living and Rebecca has a very well-paying job by any standard, so getting an apartment on her own would have been completely feasible. However, Zach was living with his brother and wanted out of that situation, so he used this as an opportunity to manipulate her into getting access to a life he can't afford/isn't willing to work for (a pattern from previous relationships). This isn't even when the trouble all started but it is where it got very real.

For a bit of context, Zach used to date a girl named Cecilia (33F) for about 4 or so years. Both Rebecca and I have known Cecilia for a long time but we were never really close with her. But we have a lot of mutual friends in common. When Rebecca and Zach started dating, those friends came forward to warn Rebecca about Zach. Letting her know he's toxic and manipulative and that she should be checking his finances because he is not an honest person. Rebecca would talk to Zach about this and give him an opportunity to explain and he would have an answer for everything. I think the conclusion was, "we just weren't well-suited. Just because things were toxic with her doesn't mean it will be like that with you." LOL

Fast forward, everything those mutual friends have come to the table with has turned out to be true (a part from the finances, as far as I know). He's extremely manipulative, emotionally abusive, and is fully isolating her from her friends. A friend asked me if he was physically abusive towards her because she saw and suspected that behavior from him when he was with Cecelia. For the record, I have seen absolutely no indication that he has physically harmed her, but it's alarming that someone would even jump to that conclusion when his name gets brought up.

I was having my own personal struggle because these two are genuinely torturous to be around. They are always bickering, fully fighting, or giving each other loaded looks that say "we'll discuss that later". I see her at the gym pretty often and she's always shit talking him (deservedly). I have never seen either one of them say a kind word about each other. She says more pleasant things about her job than her supposed partner. I'm an annoyingly empathetic person in the sense that I feel big emotions very viscerally, so it is incredibly overwhelming for me to be in an environment this toxic. I was originally planning on just telling her I needed space away from her and her shitty boyfriend.

However, I just got off the phone with another friend, Zoe (32F), who is very close with both me and Cecilia and was there to witness the entire history of Zach & Cecilia's relationship. Zoe is one of the people that raised alarm bells to Rebecca at the beginning of the relationship, and because of that, Rebecca no longer speaks to her. So I updated Zoe on how I've been feeling about this situation and said I felt like I was going crazy because I'm Rebecca's most consistent and present friend so I am the only one bearing witness to the insanity that is unfolding. She confirmed everything I was feeling and seeing was accurate and that it literally all happened exactly the same way with Cecilia and that this is a pattern for Zach. He is emotionally abusive, has no redeeming qualities, and purposely isolates his girlfriends from their friends because he knows the friends will talk shit and she'll become wise to his bullshit.

She did come to me over the summer breaking down because she felt like he completely duped her. The mask came off the second her plane landed and they moved into their new place, and it was really bad. I validated everything she was saying to me, that she shouldn't feel crazy because I've been paying attention and everything she's saying is true and real. Rebecca and I have been close friends since we were 16 so I'm pretty in tune with her energy and behaviours. I tried not to shit talk him too much throughout this and instead focused on empowering her on her decision to break up with him. Just worked on making a plan for next steps and what life could look like if she made this brave decision. Reader, she obviously did not break up with him. And she has never talked to me about their relationship again.

The actual details of what I've witnessed with my own eyeballs or have heard directly from Rebecca's mouth are just way too much to type out. I would need an audiobook's length of time. But trust, it is not good. She is planning on buying a house with him, getting married, and having kids. She did not tell me this. She never, ever talks about their relationship becuase she does not want to hear any negative criticism and she knows that's what she will get. I raised my concerns to her cousin and we were able to fill in some gaps for each other.

So, getting to the heart of the matter, how the hell do I bring this up with Rebecca? If I don't say something, literally no one else will. She surrounds herself with people who enable her and is completely unreceptive to any criticism of her relationship because of how much she was getting when she first started dating him. And he just isolates her from even spending time with her friends in meaningful ways so there isn't an opportunity to say anything.

I KNOW this isn't my business. People can choose to be in any relationship they want to be in. Just because I wouldn't want it for myself, doesn't mean it's not valid. However, I am not trying to force an outcome and break them up. I just know that when they break up (because they will eventually), I can't bear to look her in the eye and say, "I knew the whole time I just didn't want to say anything." I also recognize that by saying anything at all, it may end our friendship and isolate her even further. I don't know what the right move is.

TL;DR: How do I tell my friend she's in an abusive relationship when she is completely isolated and closed off to feedback? My intention is not to end their relationship but just to offer her some perspective so she can feel as empowered as possible while making her decisions.