You could either accept responsibility and apologize profusely and offer to replace them or continue denying he broke the pieces. Should you deny it profusely, you will aggravate the situation and cause her more distress. She's 95, after all, and is not going to change her thinking. You know he did not break them but she'll never, ever believe you.
She now doesn't want to eat with him and she refused to eat dinner last night. She told me they can't be replaced, but I'm going to look online. Thanks for your help