I'll try to summarize this in a way that makes sense.
I invited one coworker without a plus one. This co worker (co worker A) filled out their RSVP card with a plus one. I would have told them we aren't doing plus ones but their plus one happens to be my boss' daughter so....
The boss' daughter is actually dating coworker B. Would it be okay to B list coworker B so he can go to the wedding with his GF? I think that this point we will have money and space for him.
Edit: Thanks everyone for the replies. It turns out Coworker A put the wrong name on the RSVP so it is a non issue.