This is a very important question, and I don't have any easy answers either. I think the response could perhaps depend on the situation. We can't always step in to intervene, but I like to hope that we can (if given a moment alone to express it) offer hits of self-esteem (e.g., sincere compliments) or offer resources (e.g., "do you want me to alert someone / call the police?"). But I wish that domestic violence resource centers would be more public about sharing strategies for this. They would know best, but I never see them talking about it.