'Material damage to her interests' is not something she can (successfully) sue for, unless she can prove that you had a legal duty to protect her interests. Like, if she hired you to be her lawyer, and you called the landlord and told him she's a liar and he shouldn't pay her a dime. Outside of some type of contractual relationship between you, you have no obligation to her 'interests' in any way shape or form. Slander and liable only apply to statements which the speaker KNOWS to be false, not anything true or believed to be true or representing an honest opinion.