Wedding Invitations & Paper

Invitation Wording Question: Guests

Hi Everyone! I have a quick question about wording for an invite:
We're having a very small intimate wedding, and are only inviting people with a guest if they are in a serious relationship with someone that we know. It's not even necessarily a cost thing, but a space thing: It's a restaurant reception and space is limited. 

So we have a very close friend of both of ours who has been dating a great girl for almost a year now, and we plan on sending his invitation with her name on it. However, they've been having a lot of fights lately, and we're not sure that in the 10 months between now and the wedding they will still be together. She is great, but not really either of our friend. How should we word his invitation so that it's clear that she is invited as his guest, but that in the event something should happen, it's not an open "plus one" or a separate invitation for her to just come on her own? 

Honestly, I don't think it will be an issue if they DO break up. I doubt she considers herself close enough to us to just come anyway if they are no longer together, but I'm not sure if there IS some sort of etiquette here that we're missing. Do we just address it to Mr. _____ _____ and Ms. _____ _____? Or should we write to Mr. _____ _____ and his guest, Ms. _____ _____?.... 

Thanks!
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