Dear Mary: What is the etiquette of responding to save-the-dates?

Q. I have a problem with a much older friend who is slightly insecure and super-sensitive to criticism and I don’t know how to tell her an uncomfortable truth about her guest lavatory. The lavatory shaft has a coating of thick brown limescale, inches deep. She is not short-sighted so clearly both she and her cleaner think the lavatory is perfectly presentable.