One other thing that you might consider doing... this might sounds weird and/or not be comfortable, but it also might help you and them both. How about inviting them over for dinner, "We've been neighbors for years now, but I feel like I hardly know you." Talk to them, get to know them better. Find out where they're from and who they are. That may clarify personalities, abilities and responsibilities for you -- and that will also introduce you to the abused party who may come to realize that they aren't entirely alone in the world with their abuser.

I'm not suggesting you have them over to dinner and during the cheese course lean over and ask, "So which of you does most of the hitting". I'm just suggesting a non-confrontational way to understand them better and open your side of the dialogue.