Our wedding invitation was clearly addressed to a distant cousin, and he has verbally rsvp'd (via relatives -- not even directly to us with the now-overdue reply card) that he will be attending with his girflriend. This is not someone who lives with him or a long-time partner; he just is inviting her along. We were following up b/c we need to know his RSVP, so he intended his answer to be official.
We do not want to extend the invitation to her for several reasons, including the financial cost and our barely knowing him, let alone her. There are other people we'd prefer to invite, and we're not. How do we tactfully let him know his plus one isn't invited?