How can I support someone who is clearly suffering from domestic abuse but is unaware of what's going on?

This is such a difficult situation. If the person doesn't realize it's happening, they have likely been conditioned to unacceptable behavior. More so, the person who is abusing them may be playing the victim in the relationship, which, if so, makes your job in supporting this person so much more difficult. As they are caring for the abuser and feel a responsibility towards them. What can you do? Be there for them, don't judge the abuser to them, don't allow the abuser to cut off your link with the person. Research domestic abuse and have facts ready, that you can drop into general conversation with this person that may cause them to reflect. Know the organizations locally that can help, if/when they are ready to get help - you could even consider contacting a local organization to discuss options with them. From a personal perspective, I didn't know that I was in a domestic abuse relationship for a long time. I knew it was not equal, and he wasn't a good person, but I didn't know it was abusive until I left and went a therapist who told me exactly what it was, and we almost fell out over it, but over the coming days, what he told me made sense and really hit home. My abuser had cut me off from most people, so please don't allow that link to go.