Surprise significant other on rsvp

My fiance and I are planning a super intimate and small destination wedding. We are only having 20 guests and included in our invitations that we wanted to be able to spend quality time on our wedding day with those closest to us. Our venue also has a limit of only 22 people, so with 20 guests plus ourselves, we are at capacity.
My fiance has had a strained relationship with his mother for many years. In fact, they were estranged for over ten years before he and I met. In the last several years, he has started reaching out to her again and initiates plans to see her about twice a year. She does not initiate plans to see him. We included her in the wedding weekend, which consists of events we have planned and are paying for spanning three days. We chose to pay for everything ourselves and not ask for financial assistance from any of our parents. In his mother’s RSVP, she stated that she would be attending all wedding events along with her fiance. This rsvp card was the first we learned she was even in a relationship, let alone engaged. The invitation was addressed to her only because the last we had heard, she was single.
Are we obligated to include her fiance that we only just found out about in the wedding events? We have never met him, and our wedding weekend would be the first time us and others in the family have ever met this person. We would also risk issues with the venue for the wedding itself since we would then be over capacity. The other weekend events do not have the same capacity restrictions. Or should we stick to our original plans of having an intimate celebration with those closest to us, which doesn’t include a stranger we only just learned about, but risk hurting his mother’s feelings?I have told my fiance that I support whatever decision he makes and feel that he should be the one to decide since it is his mother. But he is conflicted and is seeking advice.