Is it fair to ask a friend to attend our small wedding without her long-term partner?

Lena replied, "I'd love to celebrate with you, but I won't come without Sam. I hope you understand." I was about to tell her she'd know plenty of people when my partner asked me to leave the message for an hour.

They've been together three years and live in different cities for work. We've met Sam once. We capped the wedding at fifty guests and invited partners we knew well or people who were married. That let us include my cousin's husband, whom we've also met once.

Lena hasn't demanded another invitation. She's declining hers. I keep thinking the marriage distinction sounded clear when we made the list and much less convincing when I tried to explain it to her.

We have two unfilled places, so this isn't a question of moving the walls. Do we reconsider the rule for the whole list, invite Sam, or accept that Lena won't attend? I don't want to call her difficult for taking an option our invitation left her.