should i rant or sit down/shut up?

i know that plus ones are a touchy subject because the definition depends on who you're talking to. however, my fiancé invited a friend from work. she is single, and probably late 20s (i've never met her so idk for sure). there are at least 5 other people from his work that are also invited that she would know.

i sent her invitation addressed to "Ms Jane Doe" and got the rsvp back... she added a guest. my fiancé thinks we should just let it happen. but i think we should say something. this is not the first time one of the guests has added a person and we don't have enough space for them to keep adding people.

here's my question- is this someone who should have been given a plus one already? or should we say something? and what should we say? help me lol

EDIT: my fiancé talked to her. y'all. this girl didn't even have someone in mind to bring. she just hoped she would by the wedding.... like what??