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.