Right now, it seems that you are doing everything that you can do.
Your daughter is an adult who is making her own decisions now and though you may not agree with them and they cause you concern, for now you have to step back and let her be.
As a parent, one of the most difficult things to accept is our powerlessness over our children, especially as they enter adulthood and begin to make their way in the world.
Her defensiveness shows that she herself seems aware that this relationship may not be the healthiest but for some reason she is choosing to stay with her partner for now.
All you can do is state your feelings to her and assure her that you are there for her no matter what.
She may come to her own realisation at some point that this situation is not working and decide to leave him herself.
But until that happens, she needs to know that there is support there for her without conditions or restrictions.
It is a very tough place to be, but like I said, you are doing all you can do for now. I wish you the very best,