What can we do about a change of an agreement that was not agreed upon by both parties?

A person, who was a good friend of my wife got foreclosed on her apartment and to help her out we took some of her furniture into our house, since she had no money left, no place to go and stay, and thus could save on storage fees until she had decided to keep the furniture or sell it.We had just bought a house, but had hardly any furniture, because I came over from Germany and my wife got divorced recently and left most of her furniture with her previous husband. So why not help her out?Now that person has changed completely from a rather friendly person to a really nasty person and changed as well the story and says, the agreement was that we would buy the furniture off her, which isn't true! (There is nothing in written form about that agreement)We are communicating mostly vie Email or text messages and my wife and I are willing to deliver her things to a storage facility at our own costs to end that drama. However, a couple of weeks ago my wife stained one of that person's book cases and now I am concerned that this could backfire in that the person would want some kind of compensation for having 'damaged' her book case. So what should we do? It is a rather old book case and the change of color is from a light brown to a darker brown, but the book case could have some personal value or antique value (don't think so) and her ex-friend could use that as a leeway to continue causing us trouble.