My parents never had a kind word for each other, they argued continuously for as far back as I remember. They are under stress as they are close to outliving their money. They are each in rapid physical and mental decline. I recently became aware that my mother (who has always been a very difficult person) now slaps my father when they argue. She was physically abusive when we were children, so it is how she reacts when she is angry. My father is able to defend himself, and that worries us, that he will strike back at some point. In fact, we think she may be doing it so that he will react and she can be the victim.
My mother would be considered legally competent, as would my father. There doesn't seem to be a source for advice for problems like these. Especially since my father won't admit it's happening. It must be humiliating. He isn't kind to her either, but it's hard to say who is worse in the verbal abuse department.