As an owner I would welcome the sitter bringing someone over for a visit or even overnight(s) provided we’d discussed it beforehand and I agreed. My pets are friendly with all humans and wouldn’t mind the additional interaction, though some pets are less tolerant.
As a sitter I’ve been pleased when an owner has agreed to allow a friend of family member to visit (for the day or overnight). I’m often traveling to a location where family or friends reside and I enjoy the chance to visit with them while attending to my responsibilities as a house/pet sitter.