Back story: Fiance and I decided that we'd only give plus ones to couples that were living together, engaged, or married. We both have huge families, and wanted to keep numbers under control for cost and covid reasons.
We just received an RSVP from one of my FHs cousins. He said that since his mom isn't going, he's going to bring his girlfriend instead. Not in the form of a question. Just informed us that his girlfriend, who we've never met, will be attending our wedding.
How do we tell him (without creating family drama) that she's not invited? We can't use the cost excuse, since he's essentially just swapped her in, but I want to avoid the slippery slope of "well X gets to bring his girlfriend, so why can't I?"