I invited my friend and her husband to my wedding, both their names on the invitation, not "Friend and guest." She happened to mention to me recently that she might bring this other girl instead of her husband. I was a little surprised. It's not a big deal and I don't plan on saying anything to my friend, but I'm just curious: if someone is invited with a named guest, is it considered acceptable in terms of etiquette for them to bring a different guest instead?