What I've seen in my own life is that, if you tell an abuse victim they're being abused and need to leave, they hear, "You're a moron who can't make your own decisions. You have bad taste and bad judgment. I think you're stupid."

Once I knew that, I found it easier to interact with any friends who wound up in abusive relationships.

If they were upset about how they were being treated, I'd say things like "Yeah, it would hurt my feeling if someone did that, too. I'm not sure how I'd respond, but I can see why you're upset. What do you think you should do?" You'll notice a formula:

validate feelings + state that the behavior was not okay, but without villainizing the partner + remind the person that they have agency while implying that I respect their ability to make decisions

The third part of that equation is obviously difficult when you're thinking "OMG they need to LEAVE, how can they not see it?!?!?!???" but IME it's vital.

I also offered mild positive comments about any good things their partners somehow managed to do, but I wouldn't actually praise the partners. I'd also subtly highlight that the good things weren't particularly special - that lots of people do sweet stuff, not just Mr. Asshole when he's "sorry" he called you a worthless c*** yesterday. Think stuff like "Oh, it must have made you so happy when he gave you flowers. It's always nice when guys do that!"

So far, all three friends I've talked to like that left their abusive partners. They probably would have anyway, but it felt good that I was able to be there for them without harming myself emotionally. All three came came to me to talk about how they were considering leaving in the months before they left, and I'm so glad I could be there for them.