Interesting. This is what I get from Maiden & Robustelli (p. 453): "Although Lei has the form of a feminine pronoun, the gender of adjectives (and of past participles) used with it reflects the sex of the addressee." Example: Caro professore, Lei è stato davvero gentile. "The sole exception concerns the agreement of the past participle with the direct object la (corresponding to lei, ella). In this case the past participle is generally feminine, irrespective of the sex of the addressee." Example: Dottor Biagi, l'ho vista ieri in TV. The verb è doesn't change.
lei: she. Lei with capital L is polite, usted in spanish. If you talk with a man or a lady and you want to be polite, you say Lei.
How does this sentence change when we learn that "Lei" can mean "you"? What happens to "povera"? Or is it "è" that changes to "siete"?
In that case the participle changes in accordance to the gender of the person: "Lei non è povera" when speaking to a female and "Lei non è povero" when speaking to a male.