I don't know if anyone has had experience with just showing up with an extra person- but I would advise against it especially if it changes the 'top' size of your table (ie, 3 people instead of two, or seven people instead of six) they have gotten really weird in the past couple years since dining plan about adding extra people on and those are two popular restaurants.
We ended up changing all of our ADR's for our recent trip from 4 to 5- no small feat during
free dining- and we were successful for all but one place, Le Cellier. we had never eaten there so we told the friend who was supposed to tag along with us about it- and she booked a separate dinner ressie for herself and me (I was going to take one for the team so to speak and let my family eat at le Cellier without me.)
Well, long story short the friend didn't end up going on our trip so all our ressies except that one were for five instead of four- and I did not attempt to change them because nothing was available. They never gave us a problem about having one less person- but with one more, believe it or not it's not as simple as just pulling up a chair to your table.
Why can't your freind's relative just meet them for coffee or come spend a day in the park? If they're local they would hopefully understand that especially considering how busy some times are at these places, that you can't always just add one on. When I asked Disney Dining about just showing up with an extra person, the CM about fainted and acted like I'd asked him if I could have the moon on a platter for dessert LOL. He said that he definitely did not recommend it.
I wish you the best of luck!
Laren
p.s. I would keep calling though to try to add the one person on! Try adjusting the time of your ADR a little if you're willing, or if you're able swapping days at each restaurant and you might find a table the size you need. Just be sure they don't cancel your original ressie until they've booked any new one they might say is available! The bigger the party though the harder it gets to make these alterations. Again good luck and I hope this is typed correctly I left my glasses upstairs and even with the magnifier thingie I'm blind as a bat at the moment LOL