What do you say when someone never RSVP’d to your wedding and then several days before–as in the week of the actual wedding–is like “Can’t wait til Saturday!” (Worse, I assume, would be people showing up completely unannounced, but I’m not really sure how to deal with this, either…)
I think what you probably say is “I can’t wait, either! I’m so excited you’ll be there to share our day!”
Then you call the caterer, frantically, and make sure s/he brings an extra chair and place setting. While it’s definitely not ideal to spring something like this on him or her last minute, I have no doubt that the surprise appearance AT the wedding–no response and no “can’t wait” message–is something that s/he has a contingency plan for (at least, it’s something s/he SHOULD plan for–people’s rudeness and thoughtlessness isn’t something you plan around, but it’s certainly something you can plan ON).
Yes, you could try to make sure your friend feel guilty about failing to reply with an added “thank goodness you called, otherwise I would have had no idea that you were coming, since you never responded.” But that won’t really change your current situation. And while it was rude for your friend not to get in touch, it’s possible that the RSVP got lost in transit, or that s/he was relying on a partner to get it in.
That’s why it’s fairly standard for the bride-and-groom-to-bes of the world to follow up personally with folks who haven’t RSVP’d by a week or two past the “GET THEM IN BY THIS DATE OR ELSE” deadline.
Bonus about that tactic? You get to lay on a little tacit guilt just by sending the email or making the call.