You're holding a formal event, for which you have invited social units (married/engaged/living together) together, but others who do not fall into those catagories alone. After the invites are sent one of your solo guests becomes engaged. Your invites go
Leave the guest list alone.
Extend an invitaion to the fiencee.
Send an invitaion to fiencee, and hope they don't realise they wern't initally invited, if their partener hasn't RSVP'd.
Send an invitaion to fiencee, and hope they don't realise they wern't initally invited, if the RSVP date hasn't past, even if partner has responded.